The YWCA International Fair, featuring food, crafts, music, dance and fashion from Washington embassies, is this Friday from 11 to 4 at the District of Columbia YWCA, 624 Ninth Street NW. $2.50; 638-2100 . . . "Comet Quest" -- a symposium of lectures and illustrated presentations, including "What is a Comet?" and "Soviet Space Missions to Comet Halley" -- is Friday from 1 to 5 and Saturday from 9 to 1 at the Albert Einstein Planetarium, National Air and Space Museum. Free; 357-2700 . . . The Christmas Revels -- an extravaganza featuring more than a hundred musicians, dancers and actors -- highlights Appalachian, Black American, Shaker and New England holiday traditions for its theme this year. The show is at Lisner Auditorium, 21st and H streets NW, Friday at 8, Saturday at 3 and 8 and Sunday at 2 and 7. $12.50 and $8; 657-3285 . . . The Jingle Bell Run for Arthritis is Saturday at 8 a.m. at West Potomac Park Athletic Fields and includes a 10K, a 3K Fun Run and a costume competition. Register ($10) until 8 p.m. Friday, 1901 Fort Myer Drive, Suite 507, Arlington, or on race day by 7:30; 276-7555. IN MARYLAND

The second annual "Light Up Baltimore" holiday walk, complete with carols and tree- lighting ceremonies, is Friday from 5 to 9 at Market Center. Free; 301/837-4636. IN VIRGINIA

Pianist Michael Puri performs a benefit candlelight concert to aid needy families (SHARE) Sunday at 7 at the Chesterbrook United Methodist Church, 1711 Kirby Road, McLean. $3; 356-7100.