Metamora, the trio whose singing and dulcimer-fiddle-guitar-piano-flute-concertina playing judiciously and deliciously bends -- and occasionally rewrites -- the rules of "folk" music, appears at Baird Auditorium in the Museum of Natural History this Friday at 7:30. The Smithsonian Resident Associates show is $15, $10 to members. Call 347-3030.

*Deja Vu's 10th annual Christmas in July party is this Saturday night at 8:30. The Big-Chill dance club's considerable halls -- entered from M Street at 22nd, mostly by people who remember which group Paul McCartney was with before Wings -- will be appropriately decked. Plus the boss jocks will be playing Christmas records, and Santa and his helpers will be there. We hope all of this trans-seasonal silliness helps cheer up the guys at the door, at least. No cover. Park free across M Street in Blackie's lot; call 452-1966.

*Second Story, the accomplished D.C. acoustic duo whose repertoire reflects multi- instrumentalists Meagan Lane's and Geoff de Mers' multiple interests in classical, jazz, traditional folk and downright hard-rock music, are scheduled on stage Sunday night at Flanagan's. (Flanagan's is the relatively new but quickly likable Irish pub in the Bethesda Square shopping center in Old Bethesda. Call 986-1007.)

*Root Boy Slim's backup band -- which is hardly a sufficient way to describe the otherwise front-of-the-stage rock 'n' roll band Capitol Offense, led by keyboardist Tommy Lepson -- plays the Silo Inn up in Olney, also on Sunday. To get there, take Georgia Avenue until you're sure you must be in New York State; then call 924-4242.