Trees! Music! Reindeer! It's the holidays! And here are highlights of holiday hoopla from now to 1982:

18 -- A 30-foot spruce and its 56 smaller colleagues are all gussied up at the Ellipse for the 28th Pageant of Peace. Visiting choirs and bands provide free music evenings through December 23; the decorated trees and Rudolph and his buddies are there through January 2. Visions of the Sugar Plum Fairy and the whole of the Washington Ballet Company's Nutcracker begin at Lisner. Matinee and evening performances go through January 3.

19 -- Santa and his reindeer paddle their way in at Georgetown's C&O Canal, on a raft arriving at the Foundry Mall at 1:30. The D.C. Youth Chorale and area choirs will be there to help them ashore and sing carols until 4. Free, between 30th and Jefferson streets below M. Twelve "Trees of Christmas" are at the National Museum of American History, decorated with thousands of handmade ornaments and showing until January 3. Christmas in Killarney, anyone? That's at 5601 Massachusetts Avenue, where there's Irish song, dance and food. Admission, $7.50. And the Navy Band presents Christmas music at 8 in Constitution Hall. Free; also the 20th at 3.

20 -- The story of the Three Wise Men is a bilingual play shown free at 2 at Gala Hispanic Theater at 2319 18th Street NW. And, at 3:30 that's where the traditional "Posadas" sets off: a Latin-American Christmas parade with music and a live donkey moving up 18th Street. At the Renwick, the Potomac English Handbell Ringers play classical and Christmas music at 3 and 4:15. Also free, the Cathedral Choir of Boys and Men sing at the Washington Cathedral at 4. And the Navy Band plays musical "Traditions of Hanakah" at 8 at Constitution Hall.

21 -- What's a TubaChristmas? It's when some 500 tuba, Sousaphone and baritone- horn players trot down to the Ellipse with their trusty horns to give a free concert at 8, at the Pageant of Peace.

22 -- The Nutcracker opens at the Kennedy Center, with shows through January 3. And there are poinsettias, greenery and decorations as part of an Old-Fashioned Christmas at the U.S. Botanic Garden, flourishing since December 15 and continuing through the month.

23 -- Through the rest of the month, the Kennedy Center's Grand Foyer is a grand playground, with choirs singing and handbells ringing in free shows. And the Messiah sing-along is in the Concert Hall at 8; tickets have already been distributed, but at 7:45 all empty seats will be filled.

24 -- The Christmas story is acted out in the Washington Cathedral's annual pageant, free at 4.

25 -- The Washington Cathedral has its traditional morning service, televised over NBC and open to the public starting at 10. And the Jewish Community Center of Greater Washington has a concert of Jewish folk music by the Fabrangen Fiddlers at 1. Tickets are $4.50. (Call 881-0100).

26 -- The National Museum of American History begins free entertainment that keeps up the rest of the month: American ethnic holiday foods in the lower level, singing on the first floor, choirs on the second floor, instrument demonstrations on the third floor and roving singers every day. And the Capital Hilton offers a Caribbean Christmas festival this night, 9 to 3, with West Indian music and dance. Admission's $10; call 398-1620.

27 -- A Hanukah Festival includes lighting of the menorah and the music and dance of the Jewish celebration. In the Concert Hall at the Kennedy Center. Tickets: $6. The National Geographic Society continues an exhibition open since the beginning of the month celebrating "Symbols of Faith," a look at the shared heritage of Judaism, Christianity and Islam. Free.

28 -- This might be the evening the see the Mormon Temple, lit up with 50,000 tiny white lights. Hear it, too: Free concerts have been and keep going through the monthat 7:30.

29 -- The White House by candlelight: free tours 6 to 8 starting at the East Gate. Also the 30th.

30 -- No, children, Santa isn't gone . . yet. See a child's Christmas of the 19th century at the Octagon House, displayed December 15 through 31. Or try the Museum of Modern Art of Latin America, where a traditional Peruvian nativity scene is displayed 10 to 5 Tuesdays through Saturdays until January 17. Both exhibits free.

31 -- A Night in Old Vienna at the Kennedy Center's Concert Hall at 9; tickets to hear the chamber music are $7.50, which will also get you in to the dancing in the Grand Foyer, beginning at 11 and going into the new year.--