Top off your holiday with a song and a dance. Round aboud December, people seem to be struck by "Messiah"-mania, or go nuts about "The Nutcracker," or a bit of both. Here's a roundup of the two most popular seasonal shows, free, unless otherwise noted. MESSIAH SING ALONGS * The Masterworks Chorus presents its "Messiah" Friday and Saturday at 8 at Bradley Hills Presbyterian Church, 6601 Bradley Blvd., Bethesda. Tickets are $7.50; $6.50 for students and senior citizens. 468-4172.
* If you'd rather see the "Messiah," Washington choreographer Greg Reynolds has the ticket -- his full-length ballet "Handel Dances" receives its world premiere this Friday, Saturday and Sunday at 8 p.m. at Trinity Theater (36th and O streets NW). There are also 2 p.m. performances on Saturday and Sunday. Tickets are $10 for adults, $5 for children. Call 724-4479.
* Grace United Church (25 East Second St., Frederick) presents a "Messiah" sing, featuring local singers and the Frederick Community Orchestra, Saturday at 7. 301/663-8703.
* Hah-lay-loo-yuh. Another chance to join the chorus, at the "Messiah Sing-In," Sunday at 1:30 at the New York Avenue Presbyterian Church (1313 New York Ave. NW). Seating and free parking available for 1,000 singers and listeners. 393- 3700.
* The Ntional Symphony Orchestra performs "Messiah" with the Oratorio Society of Washington Friday at 8:30 p.m. at the Kennedy Center Concert Hall. (If you want to sing along, you'll have to wait till the 23rd). Repeats Saturday, 8:30 p.m., Sunday at 2; Monday, 8:30 p.m. Tickets are $5 to $24. 785- 8100.
* Georgetown Presbyterian Church presents "Messiah" as performed by its choir, soloists, organ and orchestra, Sunday at 5. 338-1644.
* And the choirs of All Saints' Church and All Saints' Festival Orchestra perform selections from "Messiah," Sunday at 4 at the church (3 Chevy Chase Circle, Chevy Chase). Tickets are $5, available at the door. 654-2488.
* This is the big one. The annual "Messiah" sing-along atthe Kennedy Center Concert Hall features 2,800 voices (including yours, if you managed to get your free tickets earlier this month). The concert, featuring the Paul Hill Chorale, is December 23 at 8. 857-0900. NUTCRAKERS
* The Washington Ballet's annual production of the "Nutcracker" has new costumes and sets, and Tchaikovsky's time- honored tale and tunes. Performances at Lisner Auditorium, December 14 to 29, 7 p.m., with weekend matinees. Tickets $12.50 and $15. 362-4644.
* Anutter "Nutcracker": the Rockville Civic Ballet's interpretation is performed Saturday at 2:30 and 7:30 p.m. and Sunday at 2:30 p.m. at the F. Scott Fitzgerald Theater (Civic Center Park, Baltimore Road and Edmonston Drive, Rockville). Tickets are $3 adults, $2 children. 424-8000 ext. 340.