WHEN IT COMES to Christmas, there are two kinds of people -- those who live and plan for the holidays all year long (and have their house lights strung before the Thanksgiving turkey is trimmed), and those who can never quite believe it's already that time of year again.

For the former, well, they've been wondering for weeks when we were finally going to put together our annual list of the season's best -- from concerts to carol sings, Santa sitings to Christmas tree lightings. To them, we need only say, "Here. Go forth and conquer."

For the latter, this is your wake-up call. You've still got time (12 days by the shopping clock) to sample all the sights, smells and sounds of the season.

DECEMBER 13

In case you haven't heard, Christmas in Washington has officially begun -- the National Christmas Tree was ceremoniously lit on the Ellipse Thursday night, and the annual Pageant of Peace is under way. It's worth making the pilgrimage to our nation's 30-foot live Colorado blue spruce, especially since every night from 6 to 9 until December 23, school, church and community organizations from across the country will be performing for visitors at 30-minute intervals.

* Bring out the boar's head! American University is hosting its annual Medieval Madrigal Feast in Mary Graydon Center's University Club on campus at 7 p.m. The feast includes a seven-course banquet and three hours of entertainment by ye olde jesters, actors, royalty, instrumentalists and the American University Chamber Singers. $12.75. 885-3436.

* You too can learn traditional holiday decorating techniques that were all the rage in the 18th century -- including wreath designs and pine-cone arrangements -- at a demonstration/lecture at Sully plantation in Chantilly on December 13 from 10 to noon and December 14 from 1 to 3. $3. Call 437- 1794 for reservations.

* Colvin Run Mill Park's buildings are decorated Victorian- style for "An Old Fashioned Christmas" December 13 and 14 from 7 to 9. There'll be ornament-making, hot spiced cider, a visit from St. Nick, carol singing with the Mill Run Dulcimer Band, storytelling and performances by the Bluemont Morris Dancers each night at 8. The park is on Colvin Run Road in Great Falls. $3, $1 for children. 759-2771.

* The Baltimore Orioles wives club is holding its eighth annual Christmas sale of baseball paraphernalia, old yearbooks, media guides, bats, autographed balls and promotional items at Memorial Stadium's Hit-N-Run Club, December 13 and 14 from 10 to 6 and December 15 from 10 to 2. Oriole players and The Bird will be on hand. Proceeds to charities. 301/243-9800.

* Singing Scrooge: The Fredericktowne Players present "The Stingiest Man in Town," a musical version of Dicken's "A Christmas Carol," at the Weinberg Center, 20 West Patrick Street in Frederick, Md.; December 13 at 8, December 14 at 2 and 8, and December 15 and 2. $6, $5 for students and seniors. 694-8585.

* Gunston Hall is sporting an 18th-century Christmas look this year, and entertaining soldiers from the Civil War. From December 13 through 15, visitors can tour the mansion and see supper prepared for uniformed members of the First Virginia Regiment. Hours are 7 to 9 December 13 and 3 to 5 Decmeber 14 and 15. There'll be entertainment and refreshments all three days. Admission is $3, $1 for children six through 15. 550-9220.

* Masterworks Chorus is ringing in carols old and new for a candlelight Christmas concert at the following dates and locations: December 13, 8 p.m., Bradley Hills Presbyterian Church, Bethesda; December 14, 8 p.m., All Souls Episcopal Church, Chevy Chase; December 15, 4 p.m., Epworth United Methodist Church, Gaithersburg. $10. 949-1652.

* Some of your favorite holiday flicks are showing down at the Kennedy Center's American Film Institute from December 13 through 18. Here's the schedule: "Holiday Inn" and "A Christmas Carol" (1938), December 13, 8:30, and December 14, 4; "Christmas in Connecticut" and "White Christmas," December 14, 7:30, and December 15, 4; "The Bishop's Wife," December 13, 6:30, and December 16, 6:30; "Miracle on 34th Street" and "A Christmas Carol" (1951), December 15, 8, and December 16, 8:45; "Scrooge," December 17, 6:30, and December 18, 9; "Meet Me in St. Louis," December 17, 9, and December 18, 6:45. 785-4601.

* Members of A. Salon Ltd., an artists' co-op, will offer works from $1 to $100 -- paintings prints, drawings, ceramics, photographs, sculpture, jewelry and cards -- at the R Street Gallery in the old Jackson School between 30th and 31st streets NW. Thursdays from 4 to 9, and Fridays through Sundays from noon to 5 to December 21. 337-4888.

* There may not be much snow this season, but you can still go dashing through Frederick and environs in horse-drawn carriages built for two, four or more through December 31. Hours are Mondays through Thursdays, 11 to 3 and 7 to 11; Fridays, 11 to 3 and 7 to 1 a.m.; Saturdays, 11 a.m. to 1 a.m. and Sundays 2 to 10. The place to be is the Frederick Carriage and Livery, 112 Monroe Street in Frederick. $3, $2 for children under 12; $20 for a thirty-minute tour in a private (2- carriage. Call 301/695-RIDE for reservations.

DECEMBER 14

* The National Air and Space Museum's monthly sky lecture begs the question, "Was There a Christmas Star?" Find out for yourself at 9:30 a.m. in the Albert Einstein Planetarium. It's free. 357-2700.

* The National Aquarium will be giving two special Christmas tours at 10 and 2:30, with a look at life behind the glass bowl, and a visit to the Christmas tank, where some mighty talented fish will be swimming around to the music of the "Nutcracker" and other Christmas music. The Aquarium gift shop is also offering a 10 percent discount to all shoppers. Call 377- 2826 for reservations.

* If you're dreaming of a green Christmas, then check out Green Spring Horticulture Center's "Holiday Potpourri" from 11 to 3. There'll be an open house from 11 to 2, featuring the regular holiday line-up: hot cider, carolers, decorating demonstrations and fresh greens for sale. A puppet show for the '80s, "Santa's Christmas Computer," begins at 2:15. Admission is $1 for adults, 50 cents for children under 12. The center is at 4603 Green Spring Road in Annandale. 642-5173.

* The Library of Congress shows a free holiday movie for children, "A Christmas Carol" (the one with Alastair Sim), at 1 in the Mary Pickford Theater in the James Madison Memorial Building. Note: Adults must accompany children and children must accompany adults. Call 287-5677 for reservations.

* Joyful Noises: American University's Christmas Concert is in the Kay Spiritual Life Center at 2, featuring the AU Chamber Singers, Collegium and the Women's Chorus. The program includes music of the Renaissance, carols, madrigals and light Christmas melodies. $2, $1 for students. Call 885-3436.

* The Sherwood Regional Library is showing two Christmas films beginning at 3 p.m., "Charles Dickens' Christmas" and "Benjamin and the Miracle of Hanukkah." 2501 Sherwood Hall Lane, Alexandria. Free. 765-3645.

* And now, back to the Renaissance: The Arlington Early Music Ensemble presents a free concert of Christmas music performed on authentic instruments such as recorders, medieval harps, sackbuts, cornetti and krummhorns (no misspellings here) at 3 p.m. at the Lyceum, 201 South Washington Street in Alexandria. 838-4994.

* You don't have to wait for the summer to go back to the beach. Ocean City is hosting "Christmas on the Beach" starting at 4, with free horse-drawn hayrides, visits with Santa and sing- alongs with the Stephen Decatur Show Choir. We're also invited to help decorate the city's Christmas tree. The celebrations take place on the beach at North Division Street. 301/289-2800.

* The Thomas Circle Singers perform a 10th anniversary Christmas concert, including Pinkham's Christmas Cantata, Spanish carols and traditional favorites, at 8 p.m. at Luther Place Memorial Church on Thomas Circle. $8. 667-1377.

* Here's a chance to sing along with the Prince George's Philharmonic Orchestra, the Bowie-Crofton Choral Society and Con Viva Musica. They're performing works by Praetorius, Brahms and Tchaikovsky, plus traditional favorites. It's at the Queen Anne Auditorium, Prince George's Community College in Largo. 8 p.m. $10, $5 for subscribers. 699-2540.

* This Christmas party's got you covered -- carriage rides, sleigh rides (weather cooperating), walking tours of the town, music, caroling, a taffy pull, craft exhibits, free hayrides and of course, appearances by the indefatigable Saint, all in historic Boonsboro, Maryland, December 14 from 10 to 8 and December 15 from noon to 6. Boonsboro is between Frederick and Hagerstown. 301/432-8348.

* The Torpedo Factory Art Center is mixing medias and music this season, with free musical performances, from classical to folk, on the hour at various locations throughout the center, December 14 from 11 to 4 and December 15 from 11 to 4. 105 N. Union Street, Alexandria. 838-4565.

* The Alexandria Choral Society performs Part I of Bach's Christmas Oratorio, with guest conductor Robert Shaffer, at 8 p.m. at the Washington Street United Methodist Church, in Alexandria. Tickets are $14 and $9. 530-9596.

* Santa and his horse-drawn carriage will be giving block- long rides beginning at 11th and G streets NW December 14 and 21 from 11 to 3. Mayor Barry is scheduled to ride with St. Nick on December 14 at 1:30. Tickets are $1 for adults, 50 cents for children. 775-0797.

* The Folger Consort is performing a series of holiday concerts, including 15th-century English carols and the music of Isaac, Morales, Victoria and Praetorius, from December 14 through December 22 at various times in the Folger Shakespeare Library, 201 East Capitol Street SE. $11.50 evening, $10 matinee. 546-4000.

* There's no stopping Santa this year. Among other places, he's appearing at the Old Post Office Pavilion from noon to 4 on December 14, 15, 21, 22, 23 and 24, noon to 4. $4 for a Polaroid pose. 289-4224.

* A poinsettia by any other name would still score points: The U.S. Botanic Garden holds its 51st annual poinsettia show from December 14 to January 4, featuring hanging baskets of poinsettias and wreaths of living plants, and a 25-foot decorated tree. The Conservatory is at Maryland Avenue and First Street SW, and is open daily from 9 to 5. Admission is free. 225-7099.

* Eighty thousand, count 'em, eighty thousand lights will illuminate the Washington Temple of the Church of Jesus Christ of Latter-Day Saints for the annual Festival of Music and Lights. The Temple sponsors a variety of free events for the public every night through December 31, including a live manger scene beginning at 5:30, a Christmas tree display inside the visitors center, carolers on the grounds and special soloists and other musical selections inside, and the 20-minute holiday film "Mr. Kreuger's Christmas" starring Jimmy Stewart. The visitors center is open daily from 10 to 9:30. 587-0144.

DECEMBER 15

* Babes in Toyland: The Casey Community Center in Gaithersburg not only features free pictures with Santa and his sleigh, there's also a roomful of life-sized toy soldiers, Santa's Express Train, Frosty the Snowman, Rudolph the Red Nosed Reindeer and a gingerbread house. Stir everything together with some holiday cartoons, carnival games and refreshments, and you've got a mighty satisfying entertainment recipe. $2, $3 for nonresidents, and a can of food from everybody. 840-1862.

* The Northern Virginia Jewish Community Center holds its annual Hanukah Happening from 2 to 4 at George Mason Junior/Senior High School in Falls Church, including a family show with singer-storyteller Barry Louis Polisar, games for children and a lecture/discussion with Rabbi Myron Berman on "Jewish Currents on the James." The Polisar show is $3.50 for children, $2 for adults. 323-0880.

* Now hear this! Three U.S. Navy performing groups -- The Navy Band, the Sea Chanters and Country Current -- present "This Is Christmas," a free holiday concert at DAR Constitution Hall, 18th and D streets NW at 3 p.m. 433-2394.

* How Sweet It Is: the Alexandria Community Singers present their annual Holiday Dessert Concert. Eat a homemade treat while listening to holiday music at Christ Church, 118 N. Washington Street in Alexandria. $6. 822-3190.

DECEMBER 16

* The Thomas Jefferson Library is showing two animated films beginning at 2:30 p.m., "A Christmas Carol" (let's hear it for Dickens one more time) and erry Cricket," a movie about a musical cricket visiting the Big Apple. 7415 Arlington Blvd., Falls Church. Free. 573-1060.

* You just can't keep a good Claus down -- this time he's lighting Alexandria's official holiday tree at 7 p.m. at Market Square, 301 King Street. The T.C. Williams High School Choir will perform, and everyone will join in singing traditional carols. Baked goods and hot chocolate will be for sale. 838-4848.

DECEMBER 17

* Robert Bergman, director of the Walters Art Gallery in Baltimore, discusses the Nativity Scene in Medieval and Renaissance art, including the appearance in early nativity scenes of none other than Halley's Comet. It's in the auditorium of the Hirshhorn Museum. $5 for members of the Smithsonian Resident Associates Program, $6.50 for non-members. 357-3030.

DECEMBER 18

* The Banneker High School Choral Ensemble performs traditional Christmas tunes beginning at noon at the Old Post Office Pavilion, 1100 Pennsylvania Avenue NW. 289-4224.

DECEMBER 19

* Gimme that "Old Time Christmas Celebration," it's good enough for me. The City of Fairfax concludes its annual "Festival of Lights and Carols" with a grand old-fashioned affair that includes caroling by torchlight from the Fairfax County Courthouse to Old Town Hall, lighting and decorating of the city's Christmas tree (bring your own ornament), music with the Fairfax City Brass Band Choir and the Fairfax City Brass Quintet, plenty of carol singing and a yule log burning behind Earp's Ordinary to carol around. There'll also be entertainment for the kids, including a puppet show and a visit from You-Know- Who. It all begins at 7. 385-7858.

* The Paul Hill Chorale performs carols, lullabys, Christmas classics of Berlioz and Honegger, and Daniel Pinkham's "Fanfares" at the Kennedy Center December 19 at 8:30 and December 21 at 1 and 8:30. $11 to $18.50. 364-4321.

DECEMBER 20

* Here's one way to envision Christ's birth, manger and all: The Great Falls Horse Center is hosting its annual Living Nativity Pageant in the Center's barn at 4:30, with children, live animals and a brass ensemble. Hot cider and cookies for humans, and treats for the horses afterwards. 759-5505.

* Frederick, Maryland, is brightening up the night for its annual "Festival of Lights" at the New County Courthouse Square on West Patrick Street. Carols, candles and Christmas stories are on the agenda. It starts at 7:30 p.m. and candles will be provided. 301/662-5161, ext. 247.

* The 13 trees on display in the American History Museum's "Trees of Christmas" exhibit each have a theme, and here are a few just to pique your curiosity: "Christmas Mail: Delivering Holiday Cheer," "Action Santas" and "The Family of America Tree." They're on display until January 6, so come on down. 357-2700.

DECEMBER 21

* Movies for Kids: The Library of Congress features a free movie, "Scrooge" (1970), at 1 in the Mary Pickford Theater, James Madison Memorial Building. Once again, children must accompany adults and vice-versa. 287-5677.

* Movies for Kids at Heart: Martha Washington Library presents two Christmas films beginning at 2 p.m., "Charles Dickens' Christmas" and "A Christmas Carol." 6614 Fort Hunt Road, Alexandria. Free. Call 768-6700 for reservations.

* The Baltimore Consort, a group of artists in residence at the Walters Art Gallery, presents "A Renaissance Christmas Gift," featuring favorite holiday Renaissance and medieval works, at Bradley Hills Presbyterian Church, 6601 Bradley Boulevard in Bethesda. $8, $6 for students and seniors. 301/366-2684.

* The Prince George's Choral Society performs "Laud to the Nativity," a musical work by Ottorino Respighi, plus a medley of traditional Christmas carols, at the Prince George's Publick Playhouse, 5445 Landover Road in Cheverly. 8 p.m. $5, $4 for students and seniors. 277-1710.

* More from the Middle Ages: The Oratorio Society of Washington presents "A Renaissance Christmas" on December 21 at 8 and December 22 at 3 at the National Shrine of the Immaculate Conception. The audience will be invited to join in singing traditional carols. The Shrine is at Fourth Street and Michigan Avenue NE. $5.50, $7.50 and $10. 622-1773.

DECEMBER 22

* Mr. Claus takes to the ice, and we're all invited to join him at the Mount Vernon Recreation Center in Belle View from 2:30 to 4:30. He'll be giving out candy canes, and posing for pictures for those who bring their own cameras. Admission is $3, $2.25 for children and $1.25 for skate rental. 768-3224.

* The Regional Youth Orchestra Concert performs a holiday concert at the Weinberg Center, 20 West Patrick Street, Frederick, 3 p.m., $4, $3 seniors and students. 301/694-8585.

* The Cathedral Choir of Boys and Men sings a program of 17th-century French and German music at the annual free Christmas concert at Washington Cathedral at 4. The Cathe=dral is at Massachusetts and Wisconsin NW. 537-6247.

* Maybe 76 trombones can lead a big parade, but more than 100 tubas will fill the night for the Pageant of Peace at the Ellipse on the Mall. Tuba Christmas begins at 7, with Davey Marlin Jones as guest conductor. 262-8920.

* The Washington International Chorale performs a free concert of carols from many countries, accompanied by the Chinese American Music Society Orchestra and narrated by Effie Barry. The concert will take place at the National City Christian Church, 14th Street and Massachusetts Avenue NW at 7 p.m. 232-4195.

* The Annapolis Brass Quintet Concert performs traditional carols, Hanukkah music and baroque music at St Anne's Church in Annapolis. 7:30 p.m. $8.50, $5 for students. 301/268-9516.

* Lessons don't always have to be learned. And, especially at Christmastime, they're often a pleasure to listen to: The traditional service of lessons and carols will be sung at the Cathedral of St. Thomas More in Arlington at 7:30. Admission is free. 525-1300.

* La Cantata dei Pastori (Shepherd's Play) is a 17th-century Neapolitan Christmas story of music and dance by masked characters and giant puppets. It will be performed by I Giullari di Piazza (Jesters in the Square) at 8 in Baird Auditorium of the Natural History Museum, sponsored by the Smithsonian Resident Associates. $12, $15 for nonmembers. 357-3030.

DECEMBER 23

* McLean Community Center presents "Rufus, the Red- Nosed Reindog," a puppet show in which the odd-dog-out gets sent to obedience school at the North Pole. $3. 790-9248.

* Sounds of Christ, a group that performs Negro Christmas spirituals, will take the stage at 6 p.m. at the Old Post Office Pavilion, 1100 Pennsylvania Avenue NW. 289-4224.

DECEMBER 24

* Linn Barnes and Allison Hampton perform Irish and English lute music at noon at the Old Post Office Pavilion, 1100 Pennsylvania Avenue NW. 289-4224.

DECEMBER 25

*Warning: After all the pre-Christmas activity, too much exertion today could be hazardous to your state of mind. So relax, visit friends and relatives. Or better yet, stay at home, clear the wrapping paper off the sofa, and settle down for a long winter's nap. Merry Christmas!