FRANK SINATRA, who forbade distribution of John Frankenheimer's The Man/churian Candidate for 15 years, has finally re-released it through the MGM/UA Distribution Company. (It opens at the Key Friday.) Sinatra, a close friend of the Kennedy family, pulled the movie, as well as the 1954 film Suddenly, from release after the assassination of President John F. Kennedy, because both films contained story elements that uncannily foreshadowed the real-life tragedy.

Sinatra's spokeswoman, Susan Reynolds, said there have been "conversations over the years for 'Manchurian' to be distributed . . . There has been a lot of interest -- fans writing, students of film schools. It was recently shown at the {1987} New York Film Festival and received a tremendous reception. Mr. Sinatra was delighted that an arrangement was made. I don't think it was a matter of just being held. It was just recently worked out."

Friday at 8, filmmaker Henry Jaglom will introduce his latest film Someone to Love, a My-Dinner-with-Andre' variation about singles at a St. Valentine's Day dinner, featuring Sally Kellerman and Orson Welles in his last screen appearance. The film is free and will be shown at the Hirshhorn auditorium. Call 357-2700.

Ross McElwee's Sherman's March (1986), a Washington cult favorite about McElwee's desperate search to replace his girlfriend while making a documentary film in the South, returns to the Biograph this weekend, Friday through Monday. On Tuesday and Wednesday, local filmmaker Haile Gerima's Ashes and Embers (1982), a story of a black Vietnam veteran, will be shown, followed Thursday (6:30 only) by the "First Annual Washington Mountain Film Festival" -- three hours of award-winning films about the Outward Bound life. It's a benefit for the Potomac Appalachian Trail Club. $10 tickets, which can be bought at PATC, 1718 N Street NW (638-5306), include a wine and cheese reception.

Saturday at the National Gallery of Art's East Building auditorium at 2, the "Cinema of the American Avant-Garde" series continues with Bruce Conner's A Movie (1958) and Mongoloid (1977), Kenneth Anger's Scorpio Rising (1963), Bruce Baillie's Castro Street (1966), Robert Nelson's Bleu Shut (1970), and Owen Land's Remedial Reading Comprehension (1970), Institutional Quality (1969) and What's Wrong With This Picture? (1972). Call 737-4215.