Yesterday had been proclaimed Betty Carter Day in the District by Mayor Marion Barry. And, judging by the 300-plus crowd that flooded the fifth floor of the District Building for the premiere of the film, ". . . but then, she's Betty Carter," last evening, he did the right thing.

When Carter -- the jazz singer's jazz singer -- was introduced to the more than 150 people who spilled into the aisles and onto the floor of the City Council chambers (another crowd had to wait in the hall for a second showing), their welcome was deafening.

There was the same thunderous applause when Tony Gittens, head of the Black Film Institute of the University of the District of Columbia, the organization responsible for the premiere, introduced the filmmaker. "In a way, this is also Michelle Parkerson Day," he said.

The 28-year-old Parkerson, who makes her living as a videotape editor at WTTG-TV (Channel 5), accepted the ovation modestly. "I've thought since this event was planned, 'What are you going to say, Michelle, when the day comes?' And I still don't know what to say. I just want to say thank you." Brief pause. "I'm a little nervous. You all know what I mean?" Laughter and more shouts of support from the mostly black audience.

"Effi and I decided a long time ago," said Mayor Barry, after turning 4-month-old son Marion Christopher over to his wife, "that City Hall should be a place where people could come and enjoy arts and culture, as well as occasionally bring me their problems. Tonight is one of those enjoyment occasions."

Finally, the lady of the day rose to speak to the friends, known (drummer Max Roach, for one) and unknown, who had gathered to pay homage to her. "I want to thank you for making my life wonderful," said Betty Carter. "If it had not been for you and for the fact that you believed in what I was trying to say, even when I didn't know what I was trying to say . . . it's only because of you that I'm here today."

Carter left the room when the film began, saying she didn't "want to look at it and say, 'I shouldn't have said this' or 'I shouldn't have opened my mouth that wide.'"

She said that she plans to watch ". . . but then, she's Betty Carter" eventually. When she does, she will see a film depicting a black woman with an unlimited amount of talent and inner strength. A woman who started her own record company because the commercial companies "didn't have faith" in her records. Not so the crowd last night, which surrounded her at the reception following the screening, requesting autographs on programs, napkins and record albums they had brought with them. The label: Bet-Car.