The American College Theater Festival continues at the Kennedy Center with performances of "Idioglossia," a new play presented by the University of Utah in the Terrace Theater Saturday at 2 and 7:30. Then at 7:30 on Sunday, the festival's 12 regional finalists compete by presenting excerpts of their winning plays. Seating is available on a first-come basis; it's recommended you arrive at least 20 minutes before showtime. 254-3600.


The Washington Swing Dance Committee sponsors a dance with the big-band sounds of Doc Scantlin and His Imperial Palms Orchestra this Saturday at 9 in the Spanish Ballroom of Glen Echo Park. There'll be a foxtrot workshop at 8. Tickets are $5; singles are welcome. 229-4188.


There'll be plenty of barbershop music Friday and Saturday at 8 in two performances by the Alexandria Harmonizers Barbershop Chorus and the quartets Vaudeville and Copyright 86. The shows are at T.C. Williams High School, 3330 King Street in Alexandria. Tickets are $6 and $9. 248-8383 . . . Sunday is Loudoun County Day at Oatlands Plantation (on U.S. 15 six miles south of Leesburg). From noon to 5, there'll be live string bands, Scottish dancers, puppets and sheep-herding demonstrations. Admission is $4 for adults; $3 for children and seniors. Loudoun County residents are admitted free. 703/777-3174 . . . National Symphony Orchestra member Andrew Litton conducts the Arlington Symphony in a free concert of Strauss, Rachmaninoff and Tchaikovsky this Sunday at 3 in Kenmore Auditorium, 200 S. Carlin Springs Road, Arlington. 739-2903.