What was going on when I was born?

W. APPLE Jr. LONDON, April 18 - Troops and demonstrators ringing the British Embassy in Tripoli held 25 people captive for most of today, but allowed the group to return home early this evening, the Foreign Office announced. It said it welcomed the move. In London, the police still held the Libyan Embassy under siege tonight, 36 hours after the fatal shooting of a British policewoman. Intensive negotiations with the occupants of the building produced only slight signs of progress. Under the terms of the end of the siege at the British Embassy in Tripoli, the Libyan capital, members of the group, including women and children, were allowed to return home and to move freely within the city, according to the Foreign Office.

''WE were talking in the studio, wondering where Saarinen's lost credenza could possibly be,'' said Roy Slade, director and president of the Cranbrook Academy of Art in Bloomfield Hills, Mich., ''when the custodian happened by and said there was something in the basement under a tarp. It actually turned out to be the credenza.'' For seven years Mr. Slade has been searching for the original furniture in the Saarinen house at the Cranbrook Academy as part of a larger effort to restore the house, in which the Finnish architect Eliel Saarinen and his family lived in the 1930's and 40's. Recently the architectural studio at the back of the house has received the attention of the restorers. This was the drafting room Saarinen used in his practice; it also led through a rear passage to the school's architectural atelier next door. Students and faculty members often gathered there, with the drafting tables sometimes removed for larger gatherings. ''For Saarinen, conversation and socializing were important,'' Mr. Slade said. ''There used to be tea parties in this room. There was also the famous Saarinen martini.''

Walter F. Mondale appeared to have won a decisive victory tonight in Missouri's Democratic caucuses that would add to his advantage in delegates and momentum as the campaign for the Democratic Presidential nomination goes into its second half. Mr. Mondale, at a news conference tonight in a hotel near Cincinnati, said, ''It looks like we have a solid win, a good verdict in the state of Missouri.'' ''I said earlier we had a chance of getting the delegates we need by the convention in San Francisco,'' he added. ''I believe this result pushes those chances forward some more.''

Meetings of the Green District Historic Commission usually plod on routinely enough - votes allowing a sign to be built or a house to be renovated. But Tuesday night - in the matter of the First Tax District, Block 57, Tax Lot No. 2 - the commission found itself sitting in judgment on a bit of American folklore. By evening's end, historical preservationist was challenging historical preservationist. The philosophy of how history is written was being debated. And through it all, the questions that many people here have been asking privately were being shouted: Was there really a Yankee Doodle? And if so, did he come from Norwalk?

One year ago today, shortly after midday, a lone suicide driver crashed his dynamite-filled Chevrolet pickup into the front door of the American Embassy. Seventeen Americans and 46 Lebanese were killed in that explosion. But there were other less tangible casualties as well. In retrospect, the embassy bombing, while it appeared to be an isolated incident at the time, was the beginning of a sustained and successful campaign to dislodge the United States from Lebanon.

An article on Tuesday about the 1984 Pulitzer Prizes carried a listing of the prize board, supplied by the Columbia University press office, that omitted two members. They were Richard H. Leonard, editor of The Milwaukee Journal, and William J. Raspberry, columnist for The Washington Post. The Columbia announcement also misidentified the prize administrator; he is Robert C. Christopher.

FOR many, to live in Manhattan is to agree to forgo full-time use of an automobile. Most prosperous Manhattanites who own cars do not use them in the city; traffic is too frantic, vandalism too rampant, parking facilities inadequate and alternate-side-of-the-street parking rules too complicated. For Philip Slater, president of the Silver Butler Inc., Manhattan is no place to drive a car. He is one of those who uses his car for weekend spins and to take himself to a country redoubt. Recalling the the time he drove his car uptown to avoid a $5 cab fare, he says: ''I had to bribe my way into a garage with a $5 bill and then pay $8 for parking. I wouldn't dream of driving my car in town anymore.'' There are, however, those who could not face living in Manhattan without being able to turn on the ignition daily. They drive to work, pick up the kids from school and run errands behind the wheels of their autos.

Next week Advertising Age, the bible of the advertising industry and the flagship of Crain Communications, will make what could be the most risky move in its history. Hoping to expand its circulation and its advertising revenue, the weekly Ad Age will increase its frequency to twice a week. The change comes at a critical time for the magazine and its parent company. The challenge from Ad Week, a five-year-old competitor, is growing stronger. And the editorial staff, already divided by bickering between the New York and Chicago factions, is churning after the abrupt dismissal three weeks ago of its editor. Expansion Plans In addition, the company, which is considering such new ventures as the start-up of a business magazine in Detroit and the purchase of a sizable stake in the Washington Journalism Review, can ill afford a false move at Ad Age. And many of Ad Age's readers are wondering about the need for increasing the magazine's frequency.

The personal income of Americans in March posted the smallest gain in seven months and their spending rate also edged up only slightly, the Government said today, providing further evidence that economic growth is slowing. The Commerce Department said personal income rose five-tenths of 1 percent last month, the smallest increase since a rise of three-tenths of 1 percent last August. The March rate was below the February increase of seven-tenths of 1 percent and January's 1.6 percent surge in income. Even more significant, many analysts said, was the slight increase of four-tenths of 1 percent in personal consumption spending, which includes essentially everything except interest payments on debt.

The Players column in Sports Pages on Tuesday incorrectly described Harvey LaKind. He is Darryl Dawkins's business manager.

''We're just going to go out there and take the streets back from the hoodlums and give them back to the people.'' - Police Commissioner Benjamin Ward. (A1:1.)

The utility that is building the Seabrook nuclear power plant in New Hampshire laid off 5,200 workers yesterday and stopped all work on both reactors at the site, citing ''financial pressures on the company.'' Analysts and utility regulators said the move could be a prelude to bankruptcy, a possibility that the utility, the Public Service Company of New Hampshire, and its auditors have raised in recent weeks. Others said the layoffs could be the first step in canceling the entire project in a last-minute effort to keep the utility solvent. Public Service of New Hampshire said the move would save the company and its partners in the project $750,000 a day in construction costs. But analysts said that overhead, primarily interest costs on the $2.5 billion already spent, would continue to run about $1 million a day.