AMONG MANY other facets, local music makes up a large part of four outdoor events planned this weekend, Labor Day and next weekend. The events, and the musical acts scheduled to perform, include: ADD ARTS 1985 -- Sunday, noon to midnight at Gallery Place, Eighth and F NW and environs. Free. (Rain date: Monday.) Music, theater, poetry, film, dance, visual arts. Evening concerts at 8 by Julius Hemphill and the D.C. Jazz Workshop Orchestra, On Beyond Zebra and Sweet Honey in the Rock. 783-7898. LABOR DAY CHARITY CONCERT -- Monday, noon to 11 p.m., Sylvan Theater, Washington Monument grounds. Concert sponsored by the Washington Area Music Association and the Community for Creative Non-Violence. Donation: an article of dried, canned or otherwise non-perishable food. Performances by Beyond Words, the Romans, Chip Franklin, Nouveau Riche, Beatnik Flies, the Line, Dave Allen, Robbie Gordon & Threat with saxophonist Ron Holloway, City of Light, Diamond, Model Prisoners (at 7), the Dynettes, the Nighthawks and the Slickee Boys. TAKOMA PARK FOLK FESTIVAL -- Sunday, September 8, 11 a.m. to 6 p.m., Takoma Park Junior High, 7611 Piney Branch Road, Takoma Park. Free. Music starts at noon at main field stage with Vera Cruz, Eleanor Ellis, Slickee Boys, Mountain Laurel, Pete Seeger (at 3 p.m.), Rumisonko, Celtic Thunder and Rashid King & the Sounds of Africa. Performances on other stages throughout the day by Punchline, D.C. Area Feminist Chorus, Itzqueye, Ruminahui Ecuadorian Folk Troupe, Rock Creek Morris Women, Fiddlestix and Takoma Jitterbug Band. 829-8802. ADAMS MORGAN DAY -- Sunday, September 8, noon to 9 p.m., 18th and Columbia NW and environs. Free. Music on three stages continuously from noon. 18th & California: Rhythm Masters, Mamaya, Vera Cruz, the Choir Boys, the Junkyard Band, On Beyond Zebra, Premier International, Danny Gatton and the Drapes. Columbia and Belmont: Mountain Laurel, the New Keys, the Crimestoppers, Explosion, Model Prisoners, Kim Jordan & Top Flight, the Uptown Rhythm Kings, the Dynettes. 18th and Columbia: Trux Baldwin & the Starlite Orchestra, Barely White, the Cambridge Harmonica Orchestra, Jon Carroll & Metro, Big Bill Harris' Blues Revue, Machu Pichu, the Slickee Boys, Gil Scott-Heron. 462-5113.