On most days of the year, some say, the homeless are society's forgotten people. On Christmas Eve, though, they flooded out of their shelters, lifted themselves off their grates, and lugged their bags of belongings to a holiday bash thrown just for them.

Certainly, it was the promise of a wonderful meal and gifts and Christmas cheer that wooed them. Most certainly, though, it was the prospect of meeting, even just seeing, Hollywood stars and other celebrities in the flesh.

Cher, Whoopi Goldberg, Valerie Harper, Dennis Quaid, Dick Gregory, Freda Payne and others stood behind huge tables in the middle of the Washington Convention Center and served up ham, turkey, greens and other fixin's to thousands of the District's homeless men, women and children. The celebrities were bathed in the glow of television lights. The homeless were flush with excitement.

"This is the best meal we've had in weeks," said Juanita Waddell, 30, a mother of seven who lives with her children at the Capitol City Inn shelter in Northeast.

Jon O'Brien, who uses a wheelchair because of cerebral palsy and has been homeless for 13 years, had but one thing to say about the event:

"Damn good!"

Before the party, the celebrities held a news conference at the Sheraton Grand Hotel along with Mayor Marion Barry and Mitch Snyder, an advocate for the homeless and director of the Community for Creative Non-Violence, which coordinated the huge event.

They were there, the stars said, to show that people in the richest country in the world have to start working to help the less fortunate. That work includes pushing for help from the government and private sector in providing affordable housing for the poor and urging more individuals to do their part, however small.

"That's what today is all about," Harper said. "Rolling up your sleeves."

Cher, who was accompanied by her son, Elijah Blue, said the problem of homelessness is around for anyone to see, but that too many people don't realize its seriousness.

"I think people in America have the fantasy that people in the street are bums and that's not true," she said.

It could be anyone, including movie stars and journalists, who one day finds himself without a job and on the street, Goldberg said, responding to a question about the extent of the stars' past involvement in efforts to aid the homeless.

"We're here for us, for Americans," she said.

Activist Dick Gregory, asked for his views on the Reagan administration's handling of the homeless problem, said it would still exist if Democrats were in the White House, even "if my mother was in the White House . . . . This is a national problem, an American problem," he said.

The celebrities, led by Snyder, then walked a few blocks to the Community for Creative Non-Violence shelter at Second and D streets NW, where they were given a tour.

"That really is her," a man shouted when he saw Cher.

Snyder led the celebrities through the shelter's living areas, where men were resting on their beds, and through an outpatient clinic and the kitchen. Cooks there were heating up and slicing hams and turkeys for the big party later in the day.

Outside, while autographs were being signed, Ronnie Jones, 32, a shelter resident, was in awe. "It's really nice to see that they care," he said.

Snyder's shelter was not the only one that received an unusual visit yesterday. At the Capitol City Inn and the Pitts Motor Hotel in Northwest, a man in a Santa hat passed out toys and led children in Christmas carols.

"Who's that?" said a small girl, barely able to see over a table top.

"The mayor," her mother said.

Rockcie Nichols, 9, who lives at Capitol City with her five brothers and sisters and her father, Jamison Nichols, said she wanted a bicycle for Christmas and a house, "a big one," where her family could live.

Her father, 37, who moved the family here in September from Oklahoma City is raising the children alone. Comptex, a local social service program, is helping him get back on his feet.

Gail Knight, 23, a mother of five with a sixth on the way, has lived at Pitts for almost a year. Christmas, she said, would have been just another day if not for the toys her children received from the mayor.

"I haven't seen a Christmas like this for a long time," she said, propping her youngest, 10 months, on her belly. "I really appreciate this for the children. Thank God they got something this year."

According to the city's estimate, there are 6,000 homeless people in Washington. Snyder and other advocates say there are between 10,000 and 15,000. Tickets to the Convention Center party were given to thousands of people in shelters and on the streets, and Snyder said he estimated that 3,500 attended.

That figure, he said, debunks the myth that homeless people just want to be left alone. "When you extend a serious invitation, they grab it just like that," he said.

The Convention Center was filled with Christmas decorations, round tables, long lines, music and the aroma of food donated by a host of corporations and individuals.

Gil Scott Heron sang of life's down side and actor Quaid and his band, the Eclectics, planned to make their debut. Quaid wrote a song, "Homeless Frame of Mind," for the event, he said. Johnny Rivers and his band, singer Phoebe Snow and the Fabulous Thunderbirds also were scheduled to perform.

"The people here, they're committed," said Jack Valenti, president of the Motion Picture Association of America, said of the actors, singers and other celebrities who attended. "I think if there's anything that really breaks your heart it's people sleeping on the street and who don't have a place to go."

Christine Fox, 34, was one of the beneficiaries of their efforts. She took her two small children, a boy and a girl, to the party. While they ate heartily, she spotted a familiar face from "Hill Street Blues," actor Joe Spano, who played Lt. Henry Goldblume.

Snatching up one of the party's green programs, she ran over for an autograph. She studied his signature, then looked up and said, "Can I get a Merry Christmas hug?"

She got it.