The date of the holiday open house at Oxon Hill Farm was incorrect in yesterday's Weekend section. The event is Sunday from 1:30 to 3:30 p.m.

Synchronize your calendars. The countdown to Christmas is under way, and it won't be long before the Eleventh Day becomes the Eleventh Hour.

The holiday season encompasses a multitude of events -- the colorful and commercial side-by-side with the sentimental and spiritual. To help you sort them all out, we've made a list and checked it twice. The rest is up to you. DAY ELEVEN (FRIDAY)

* Thursday night, President Reagan pressed the button and lit the lights of the National Christmas Tree, opening the 31st annual Pageant of Peace on the Ellipse. Evening musical programs, featuring choirs, bands, bellringers and gospel groups, will be held nightly from 6:30 to 9 through December 23, and the trees will remain lighted through midnight January 1. Call 426-6700 for a pageant schedule.

* The hallowed halls of history have been decked with holly, etc. The National Museum of American History opens "The Trees of Christmas," an exhibit of a dozen decorated spruce and fir trees. Among the this year's trees: "French Ribbon," "Counted Embroidery," "Shell Tree," "Teddy Bear," "Japanese Origami" and "Weaver's Tree." You can't miss them -- they're near the pendulum. 357-2700.

* "Alien Equation" is a space fantasy performed by a dozen young area actors as part of the Kennedy Center's Holiday Festival. Performances begin at 7:30 in the Theater Lab, with repeats Saturday at 10:30; Sunday, 2; December 21, 7:30; December 22, 10:30 and 12:30. 254-7168.

* Celebrate, Victorian style, at Colvin Mill Run Friday and Saturday, 7 to 9. St. Nick will be there, the kids can make ornaments, and everyone can drink hot cider by the bonfire. Admission is $3 adults, $1 children. 703/631-1172.

* Carols sound better by candlelight at Gunston Hall, the Lorton home of George Mason. The 1st Virginia Regimentwill be abiding in the fields. Friday, 7 to 9; Saturday and Sunday, 3 to 5. Admission $3 adults, $1 kids. 703/550-9220.

* Seeing stars: The Arlington Planetarium will show how the night sky looked 2,000 years ago. A program called "The Star of Bethlehem" shows Fridays and Saturdays at 7:30 and Sundays at 1:30 and 3 through December 23. Admission is $1; call 558-2868 for reservations.

* The yule log's mostly just a memory, but one burns bright at the Vansville annual Yule Log Festival beginning at 8 at the Vansville Neighborhood Recreation Center, 11733 Old Baltimore Pike, Beltsville. 301/445-4500.

* "Joy" to the world: The Troupe, a young people's performing arts company, presents the premiere of "Joy," a musical comedy, Friday and Saturday at 8:30 at Reston Community Center, 2310 Colts Neck Road, Reston. 703/476-1111.

* "A Celtic Christmas" is the theme of a lecture/demonstration of music and instruments from Ieland, Scotland, Wales and Brittany, by Lois Kuter, an ethnomusicologist (say that three times fast) and teacher at Catholic University's Continuing Education Program. The music begins at 8 at CU's John K. Mullen Memorial Library. 635-5488.

* Small world, isn't it? The National Geographic Society opens a new exhibit -- "The Enchanting World of Dollhouses and Miniatures" -- through February 17 in Explorers Hall, 17th and M streets NW, for a nostalgic look at toys before the microchip era. 857-7000.

* The nuit before Christmas: American Film Institute shows "Christmas Holiday," a 1944 murder mystery with Gene Kelly and Deanna Durbin, with "Born to Kill," another film noir oddity, Friday and Saturday, 8:30. Nonmembers $4. 785-4601.

* "A Musical Christmas Card," with a sing-along, is presented by the Fairfax Choral Society, Friday and Saturday at 8 at Fairfax Presbyterian Church, 10723 Main St., Fairfax. Tickets are $6 for adults; $4 senior citizens and youngsters. 703/442-8212. DAY TEN (SATURDAY)

* Seasick Santa? Mr. Claus will arrive in Washington aboard the Canal Clipper and will disembark at 2:30 at Foundry Mall (30th and Thomas Jefferson streets, below M Street on the C&O Canal). The Trinidad Steel Band and the U.S. Navy Ceremonial Band will greet St. Nick. 472-6685.

* "Holiday Expo '84" is the fifth annual show of Washington's Afro-American artists, craftspeople and designers, open noon to 9 Saturday and Sunday at the Washington Convention Center, Ninth and I streets NW. Admission is $2. 546-7651.

* Feet First Holiday Extravaganza of Jazz and Tap Dance, 3 to 5, Northwood High School, 919 University Boulevard West, Silver Spring (656-9076).

* "Hansel and Gretel" is a young people's opera written by Englebert Humperdinck (the 19th-century composer, not Tom Jones' rival) featuring the Sandman, dancing cookies and someone called the Dew Fairy. Performances Saturday at 2 and 4 at Fairfax High School, 3500 Old Lee Highway. 828-8118.

* The Folger Consort's annual holiday program is called "Sing Heigh, Ho! Unto the Green Holly" this year, featuring music of the Middle Ages and Renaissance. Performances run from Saturday through December 23 at the Folger Shakespeare Library. Tickets are $11.50. Call 546-4000 for performance times.

* The poinsettia, the traditional Christmas flower, was named after Joel R. Poinsett, who introduced it to the U.S. from Mexico in 1830. That's a point of fact from the Botanic Gardens' annual poinsettia display, open through January 1, 9 to 5 at First Street and Maryland Avenue SW. 225-6670.

* The good Scouts of Troop 1946 will hold a tree sale Saturday and Sunday, 9 to 5 at National Presbyterian Church, Nebraska Avenue and Van Ness Street NW. 244-6084.

* Kid stuff: Santa will have a session on making gifts with natural materials, 10 to noon at Hidden Oaks Nature Center, 4030 Hummer Rd., Annandale. Admission is $2, including all materials. 941-1065.

* And at Mount Vernon Recreation Center, 2017 Belle View Boulevard, Alexandria, kids six to nine can learn how to make ornaments from bread dough. Cost is $3, and children should bring their favorite cookie cutter (besides Mom). Call 768-3222 for reservations (deadline is this Friday).

* Brunch has become so fashionable in recent years that even the Clauses are coming. The Good Luck Community Center, 8601 Good Luck Rd., Lanham, holds a brunch for Santa and Mrs. C, with music, dancing and movies from 10 to noon. Cost is $1.50 per child, one parent admitted free. Call 551- 1093 to register.

* Dreaming of a green Christmas: The Brookside Gardens conservatory's annual Christmas display (featuring two 12-foot poinsettia trees made from more than 400 plants) is open every day except Christmas, 9 to 5, at 1500 Glenallan Ave., Wheaton. 301/949-8230. Landon School, 6101 Wilson Lane, Bethesda, will sell fresh-cut trees, wreaths, holly and poinsetas Saturday, 10 to 5. 320-3200. The St. John's Church Greens Sale is 10 to 5 at 3240 O St. NW. 338-1796. And the Green Spring Farm Park Horticulture Center, 4603 Green Spring Road, Annandale, will be selling natural wreaths, door swags, kissing balls and poinsettas, 11 a.m. t0 3 p.m. Santa has agreed to show up for this, too. It'll be a miracle if he makes it through his rounds. 642-5173.

* The Kutch Edwards Orchestra calls the tunes for the Christmas Dance, 8:30 to 11:15 at the National Prebyterian Church Fellowship Hall, Nebraska and Van Ness NW. Admission is $5. Call 649-3079.

* Last two days to catch "Cricket on the Hearth," an adaptation of the other Charles Dickens holiday tale, Saturday and Sunday, 1:30 and 3:30 at Adventure Theater, Glen Echo Park. Tickets are $3. 320-5331.

* Small tales: stories of the season will be told every Saturday through December 22 at 10:30 at the main stage of the Pavilion at the Old Post Office, 1100 Pennsylvania Ave. NW. It's sponsored by the D.C. Public Library. Call 289-4224.

* Santa swings by Long Branch Nature Center, 625 S. Carlin Springs Road, Arlington, 10:30 to 12:30. 845-7640.

* The five-member Washington Pro Musica ensemble will perform Renaissance and modern settings of the Christmas texts "O magnum mysterium" and "Hodie Christus natus est," at 8 at Dumbarton United Methodist Church, 3133 Dumbarton Ave. NW. Tickets $6.50. 549-4622.

* Sea-sons greetings: The National Aquarium (in the Department of Commerce Building, 14th and Constitution Ave. NW) will offer behind-the-scenes tours at 10 and 11 and 2:30. And there's a tank in which fish "dance" to the Nutcracker Suite. Reservations: 377-2826.

* Carols from Germany will be sung by candlelight by the Deutscher Klub of Northern Virginia Community College, at 7:30. German desserts will be available afterwards in the Tyler Building of the Alexandria Campus, 3001 North Beauregard St., Alexandr. Call Amy Goss, 845-6207.

* Oxon Hill Farm's annual holiday open house is from 1:30 to 3:30, with horse-drawn wagon rides, hot cider, caroling, and a visit from the omnipresent St. Nick. Bring a homemade ornament for the tree, or you can make one at the farm, just west of Maryland Rt. 210 on Oxon Hill Road). 301/839-1177.

* Kids in Alexandria will get a chance to talk to Santa or the Mrs. by phone between 10 and noon, through a special arrangement made by the Alexandria Department of Recreation, Parks and Cultural Activities. To get a callback from one of the Clauses, call 838-4343, 838-4843 or 838-4844. DAY NINE (SUNDAY)

* Talk to the animals -- the National Zoo invites kids to bring an ornament made of animal food to decorate a tree for the Zoo's wild birds and squirrels. Carols, Santa, cookies and cocoa, etc., 1 to 1:30. Call 673-4717.

* Jazz it up: New Playwrights' Theater tosses a Jazz Evening with ivory tickler John Eaton, who is likely to make a concession to the season and toss in a carol or two, at 8 at the Wintergarden of the Embassy Row Hotel, 2015 Massachusetts Ave. NW. Tickets are $35 per person, including buffet. 232-1122.

Hanukah Family Festival, the Jewish Community Center's annual gala, is 1 to 4:30. Along with the traditional dreidl games, cookie-baking and menorah-making, in special children's concerts, an orchestra will play "Carnival of the Animals" and the "William Tell Overture" and the Curtain Up senior adult theater group performs Sholem Aleichem's "Melamed the Goat" at 1:30 and 3. 881-0100.

* "Lessons in Carols," a choir recital, will be offered at 7 at St. Mary's Catholic Church (Rt. 450 and Ardwick-Ardmore Road, Landover Hills). 301/772-7620.

* Candlelight Christmas Concert and Carol Sing, at 8 at Fairfax United Methodist Church, University Drive and Stratford Ave., Fairfax City (703) 591-3120.

* Washington's own Paul Hill Chorale presents its annual Christmas Candlelight Concerts, this year featuring folksinger Cathy Fink, at 8:30 in the Kennedy Center Concert Hall, with repeats Wednesday and Thursday and a family matinee December 22 at 2. 365-2770.

* Seasonal sounds from Handel to Count Basie are on the bill at the University of the District of Columbia's Christmas concert, with the UDC Chorale and Jazz Ensemble, at 5 at the UDC auditorium, Connecticut and Van Ness NW. 282-7575.

* Nautical Noel: The U.S. Navy Band, Country Current and Sea Chanters Chorus presents "A Christmas Celebration" at Constitution Hall, 18th and D streets NW. Doors open at 2, concert begins at 3. 433-2525.

* This is for the birds: Fairfax County Park Authority naturalists will be making a wild feast from 2 to 3 at Hidden Pond Nature Center, 8510 Greeley Blvd., Springfield. Dress warm and help make suet cakes, pinecone birdfeeders, popcorn strings and other wildlife snacks. 451-9588.

* If you can't find it at the National Capital Winter Maket, forget about it. The antiques and collectibles flea market will be held from 10 to 5 in the indoor pavilions of the Montgomery County Fairgrounds in Gaithersburg. Admission is 50 cents, kids free. 301/490-0112.

* The indefatigable Santa is stopping by Fort Washington Park at noon to decorate the fort in the style of the Civil War days. Everyone's invited to help. 292-2112.

* His Jolliness moves on to the Gulf Branch Nature Center, 3608 N. Military Road, Arlington, 12:30 to 3:30 for a lapsitting session. For older kids, there are antique trains. 558-2340.

* The McLean Orchestra's annual family holiday concert features the Mclean Choral Society performing Rossini's "The Fantastic Toy Shop," Bach's Cantatas for Advent and Christmas, and Fred Waring's arrangement of "The Night Before Christmas," among others. Shows at 2:30 and 5 at the Alden Theater, McLean Community Center, 1236 Ingleside Ave., McLean. Admission $7, $4 for stunts and senior citizens. 281-8702.

* Your mission: to help the Civil War Santa and the men in the garrison at Fort Washington Park, Maryland prepare for Christmas. There will be games, singing and decorating from noon to 5. 292-2112.

* Evergreen ideas: A National Park Service naturalist leads a two-hour hike in search of plants that don't go dormant. Leaves the Rock Creek Park Nature Center at 9:30. 426-6829. DAY EIGHT (DECEMBER 17)

* The Holiday Performance, an annual assemblage of actors from area theaters, presents a eading of J.M. Barrie's "Peter Pan" at 8 at National Theater, 1321 Pennsylvania Ave. NW. Admission is 10 pounds of non-perishable food (collection begins at 7). 783-3370. DAY SEVEN (DECEMBER 18)

* You know the words: Norman Scribner leads the Choral Arts Society of Washington in traditional carols and seasonal music, and ends it all with a big singalong; at 7 at the Kennedy Center Concert Hall. Repeats December 21 at 8:30 and December 23 at 3. Tickets are $5 to $18. Call 244-3669.

* All dolled up: The Washington Dolls' House and Toy Museum, 5236 44th St. NW, has opened its annual Christmas toy exhibit, along with the permanent collection of antique dolls and houses, toys and games. It's open Tuesdays through Saturdays, 10 to 5, Sundays noon to 5. 244-0024. DAY SIX (DECEMBER 19)

* Families are welcome to attend the Madison Community Center's annual holiday program from 7 to 9 at 3829 N. Stafford St., Arlington. 527-6771. DAY FIVE (DECEMBER 20)

* Santa in the spotlight: The annual musical sponsored by the Department of Recreation and Howard University is called "Santa Goes to Cabbage Patch Land." Performances Thursday through Saturday at 1 and 7:30 at Cramton Auditorium, Sixth and Fairmont St. NW. Free. 673-7660. DAY FOUR (DECEMBER 21)

* Frederick's annual Festival of Lights, with candlelight caroling and carriage rides, will begin at 7:30 in the courtyard of the new City Hall. 301/663-8703.

* Folkies David and Ginger Hildebrand lead carols at 7:30 at Ellanor C. Lawrence Park in Centreville, Virginia. Bring an ornament for the park's tree. 703/631-0013.

* The Oakdale Emory United Methodist Church (Georgia Ave. and Emory Road, Olney) presents its 21st annual Living Nativity Thursday through Monday. The performances involve more than 50 people (with cameo appearances by real animals) and will be repeated at 7, 7:30, 8 and 8:30 and on Christmas Eve at 7:30. 744-4666. DAY THREE (DECEMBER 22)

* Celebrate Hanukah at the Kennedy Center Concert Hall -- the Shir Chadash Chorale will perform Yiddish, Hassidic and Ladino songs and excerpts from Handel's "Judas Maccabaeus." Plus children's choirs and dancers and the traditional lighting of the Hanukah candles. 857-0900.

* He's a notoriously big eater, but there should be enough for everyone at the Breakfast with Santa at 9:30 at Manayunk Tavern, 207 W. Patrick St. and The Province, 131 N. Market St., both in Frederick, Maryland. 301/662-0373 or 301/663- 1441 for reservations. Santa will also be eating breakfast from 9 to 11 with kids age three to eight at Tucker Road Community Center, 1771 Tucker Road, Fort Washington, Maryland. Admission is $1, reservations required. 248-4404. DAY TWO (DECEMBER 23)

* Strains of Christmas music from the 17th, 18th and 19th centuries will echo through the old Sully plantation in Chantilly, Virginia, at 2 and 3. 703/437-1794.

* Kids 12 and under get their own Christmas party -- including a play and a visit from Santa -- from 4 to 7 at the Seat Pleasant Activities Center, 311 68th Place. 336-1184.

* Free music from the Renaissance to the Baroque periods at Washington Cathedral at 4. The cathedral choir and chamber orchestra will be joined by two organists, a harpist and a partridge in a pear tree (just kidding). 537-6247.

* The traditional Anglican Service of Lessons and Carols for Christmas is a pageant of song and sight, including seasonal music from many historical periods. The Service will be conducted at 7:30 p.m. at the Cathedral of Saint Thomas More at Arlington Boulevard and Glebe Road, Arlington. 525-1300. DAY ONE (DECEMBER 24)

* 'Twas the day before Christmas, and 'twas heard live from the Kennedy Center Concert Hall over WMAL-AM (630). The free five-hour event features host Tom Gauger, with lotsa local folks, including the Montgomery County Youth Symphony Orchestra and Chorus, the Chantilly High School Band and "Jazzin' " Ensemble, a Scottish pipe and drum corps and more. It runs from 10 to 3; come as you are, when you want.

* Ring them bells: The handbell ringers of St. Matthews United Methodist Church will do their stuff at noon at the Air and Space Museum's Milestones of Flight Gallery. 357-2700. CHRISTMAS DAY

* Merry Christmas! Today's events include gift unwrapping, family reunions, visits from friends, turkey and ham dnners, December 25, all day, all over town. THE DAY AFTER (DECEMBER 26)

* The Smithsonian kicks off its annual Holiday Celebration at the National Museum of American History, with more than 70 performers offering special programs from noon to 4 daily, through December 31. 357-2700.