One day last month, Sean Doherty, 22, of Cleveland Park, spent the night holding a lighted candle as part of a silent protest outside the Iranian Embassy. Yesterday morning, he hung a large American flag from his front porch.

Across town, on Capitol Hill, Barbara Bell, 46, and her family sent a Christmas card to the 50 American hostages in Tehran Monday, and yesterday she draped her flag from a second-floor bedroom window sill.

Doherty and Bell were among an uncounted number of area residents who observed National Unity Day yesterday by heeding President Carter's proclamation that asked all Americans to display the flag to symbolize nationwide support for hostages now in their sixth week of captivity in the American embassy in Tehran.

At the Capitol, a number of flags were placed outside congressional offices.

While apparently only a comparatively small number of private citizens displayed the flag, Metrobuses flew tiny flags, flag dealers reported sharp increases in sales, and one of the city's most popular radio stations asked listeners to telephone in additional suggestions for demonstrating national unity.

"It's such a heartbreaking thing that's happened to the hostages and their families," said Bell, a housewife. "It really does take some of the glow off Chirstmas."

"I hope it shows Iran and the world that we really are pulling all together," she said, explaining why her flag was displayed.

Doherty, who was discharged from the U.S. Air Force nine months ago, said he wanted to demonstrate "that I'm behind the idea of getting the hostages out safely and as expeditiously as possible.

John Robert Webster, 18, of Chevy Chase, attached his small flag to the rear view mirror of his 1970 navy blue Cadillac. "The church down the street from me rings its bells all the time for the hostages," he said. "They're doing their thing, now I'm doing mine."

"Brad McMahon, 23, a consultant on political campaigns, said he and his four housemates put up red-white-and-blue bunting immediately after they heard about National Unity Day on a Monday night news program.

It was also a brisk sales day for most flag dealers.

"They're buying flags like they were going out of style," said Thomas Garrison of Garrison's Toys in Rockville.

He said he had sold 150 small flags and 15 large ones by mid-afternoon yesterday.

"People are flag conscious," he said. "I think the public is fed up with what's going on" with the loss of American prestige in Iran and elsewhere in the world.

"It's been an unusual sales day," said Claude Haynes of the National Capital Flag Co. in Alexandria.

Haynes said flag sales had been slow since the 1976 Bicentennial but yesterday, he sold about 150 flags, almost four times the average number of sales in a day.

Some variety store retailers said they had turned away many requests for flags because they had none in stock.

But by and large, area residents appeared to have ignored the day.

"I wish more people had put out flags," said Bell, "I don't think it's not that they don't care, but here in Washington they might think it's hokey. People in D.C. are funny."