Frank Cassidy, special events manager of the Kennedy Center public relations department, died yesterday morning of an apparent heart attack in his Northwest Washington home. He was 57.

Mr. Cassidy's death came a few hours before the annual press conference of the American College Theater Festival, of which he had been executive director for five years.The post is now held by David Young, but Mr. Cassidy continued his interest in the project.

Informed of Mr. Cassidy's death just as he was taking a plane for New York on business, Roger L. Stevens, chairman of the Kennedy Center, expressed "great shock and a profound sense of loss. Frank," he said, "was an unusual, sensitive man."

Mr. Cassidy seemed more a man of academe than of theater's hurly-burly. He often appeared to be smiling quietly to himself; spoke in almost a gentle whisper and never was one to put himself forward.

But under his auspices rather a snocker of a play, "Live Like Pigs," introduced England's John Arden to the New York theater. With Circle in the Square's Theodore Mann and Howard Zucker, thid was produced in 1965.

Born in New York, Mr. Cassidy won a scholarship to Cornell, which he promptly lost when he took to compus theater acting. He once remarked: "I was in seven failures in two seasons on Broadway. Besides that, I played teen-agers to the age of 33, which grew rather tiresome. All that led me to believe that I was not cut out to be an actor.

"I never once sat in an audience and said to myself, 'I could play that role better than he does.' But I have said to myself, 'I could produce this show a lot better than it's being done here."

So he took to directing and managing regional theaters in Plymouth, Mass., Traverse City, Mich.; Stamford, Conn.; Lakeland, Fla., and Dobbs Ferry, N.Y.

After his years with New York City's Circle in the Square, he became producer of the Theater Company of Boston. In 1970, he was named executive producer of the American College Theater Festival, a post he held for five years.

That meant 20,000 miles of travel annually to hundreds of American colleges and universities. But travel he was used to. In his acting days he had even been with a British repertory company touring India.

The quiet Cassidy style was recalled yesterday by Harry Bagdasian, founder and producing director of the New Playwright's Theater of Washington: "Frank was one of the first five people to contribute to NPT when we were just thinking of trying to start. He had the faith and passed it along."

Mr. Cassidy is survived by his wife, Jeanne Tufts Cassidy.

The family suggests that expressions of sympathy may be in the form of contributions to the Friends of the Kennedy Center, 2700 F St. NW, Washington, 20037.