Here's a day-by-day rundown of ho-ho-hos from now till Tuesday, December 30. 19 -- This is a big day for Christmas fun. The SMITHSONIAN'S MUSEUM OF AMERICAN HISTORY has lunchtime concerts, at noon and 1, to herald the opening of "Trees of Christmas" show. The NATIONAL GEOGRAPHIC SOCIETY CHORUS serenades at 3 in Explorers Hall, and at the Kennedy Center, the PAUL HILL CHORALE presents a candlelight concert at 8:30. Old TOWN ALEXANDRIA comes alive at 7 for a candlelight tour of historic homes, replete with string quartets, fifes and drums. The tour is repeated on the 20th. And the Chevy Chase Presbyterian Church stages a free performance of Menotti's Amahl and the NIGHT VISITORS at 8. 20 -- THE U.S. NAVY BAND gives a free concert, "Christmastime Around the World," at 8 in DAR Constitution Hall, with a repeat performance the 21st at 3. A SEASON TO BE, an improvisational look at Yuletide by a children's acting company, comes to the Kennedy Center's Theater Lab with free performances at 11:30 and 1:30, and again on the 27th at 1 and 3. The CHORAL ARTS SOCIETY of Washington sings in the Concert Hall at 8:30, and again at 3 and 8:30 the 21st. And the TAKOMA MANDOLEERS offer a free concert at 2 in the Museum of American History. For a do-it-yourself songfest, there's CAROLING THROUGH THE METRO, starting at noon at Metro Center. 21 -- THE CHILDREN'S RADIO THEATER presents a kids' show, "The Clown of God," at 1 in the National Zoo's Education Building. At 3, the NEW HOPE BAPTIST CHURCH SINGERS can be heard at Union Station's National Visitors Center. And Corelli's Concerto for CHRISTMAS EVE is on the bill at the National Gallery of Art's free concert at 7. 22 -- MERRY TUBACHRISTMAS -- 300 tuba, sousaphone and baritone-horn players -- comes to the Pageant of Peace at 6:30 on the Ellipse. THE PAGEANT, featuring a pet-a-zoo, nine reindeed and the National Christmas Tree, started the 18th and runs till January 1, with musical programs through the 23rd. Nighttimes through the 24th, the NATIONAL NATIVITY DRAMA, with 80 actors and live animals, stages a 40-minute reenactment of the nativity at 7, 8 and 9. 23 -- At the Kennedy Center, the Paul Hill Chorale leads a free MESSIAH SINGALONG in the Concert Hall at 8, while A partridge in a pear TREE, a new play starring James Mason and Clarissa Kaye, opens at 7:30 in the Eisenhower Theater. 24 -- The American Ballet Theater opens with Tchaikovsky's The NUTCRACKER, choreographed by Mikhail Baryshnikov, at 2 in the Kennedy Center's Opera House. And the WASHINGTON CATHERDRAL presents the traditional Christmas Pageant at 4. 25 -- Christmas Day. The Washington Cathedral's nationally televised NATIVITY CELEBRATION begins at 10 can can be watched on Channel 4. And the Jewish Community Center of Greater Washington presents Go CLOSE THE DOOR, A JEWISH FOLKTALE, with life-size puppets, at 1:30. 26 -- The Smithsonian launches a gala HOLIDAY CELEBRATION at the Museum of American History, replete with handbell ringers and string bands, continuing till New Year's Eve. At the Museum of Natural History, today and the 27th, it's A SOUTH AMERICAN CHRISTMAS, with Andean folk singers and craft demonstrations. And tonight might be a good time to check out the doings at the MORMON TEMPLE VISITORS CENTER in Kensington Choirs from around the country have been giving concerts all month, starting at 7:30. 27 -- The National Theater offers a free musical, The night befo' CHRISTMAS, at 9:30 and 11 in the morning. 28 -- The Washington Cathedral comes across with a Jubliee festival CHROUS CONCERT at 4. And the annual Christmas Hike starts at 3 at the Prince William Forest Park Nature Center. 29 -- The White House, dressed in its Christmas best, heralds the season with CANDLELIGHT TOURS from 6 to 8, and again at the same time on the 30th. 30 -- if you can't bear sneaking those useless gifts back whence they came, check out the CHRISTMAS COOKIE TREE in Explorers Hall, or the final throes of the annual POINSETTIA SHOW at the National Botanic Garden.