What was going on when I was born?

The gross national product, the broadest measure of the nation's economy, declined at an annual rate of 2.5 percent in the final three months of last year, the Commerce Department reported today. For 1982 as a whole, it fell 1.8 percent, the sharpest year-to-year decline since 1946. The department attributed much of the fourth-quarter decline in the G.N.P., which measures the total value of goods and services, to a drop in exports and to manufacturers who slowed production and cut inventories in the face of discouraging sales. These declines offset rises in consumer spending and housing.

Could the recovery turn out to be appreciably stronger than almost everyone expects? It has happened before. Although predictions for an anemic recovery still predominate, some economists foresee a somewhat snappier comeback. ''I think people are going to be surprised about the strength of the recovery as we move through the year,'' said Edward Guay, chief economist for the Cigna Corporation, the insurance company. He predicted that real gross national product in the fourth quarter of 1983 will be 5.8 percent higher than in the last quarter of 1982, and that industrial production will be 8 percent higher. He said that average annual real G.N.P. in 1983 will be 3.6 percent above 1982. ''Forecasters, by and large, have usually underestimated the speed of recovery once it gets started,'' stated Edgar R. Fiedler, vice president for economic research at the Conference Board. ''It is often hard to see the sources of strength. But once they get started, they tend to cumulate even faster than expected.''

Mayor Koch chose a celebration at a West Side synagogue last night to complain that, among religious institutions providing beds for New York City's homeless population, ''there is not a single synagogue.'' ''We all know that the Jewish community could be doing more to fulfill its responsibility to the quality of life in New York,'' the Mayor said in a speech honoring the 75th anniversary of the Stephen Wise Free Synagogue, 30 West 68th Street. He then renewed his recent theme that a troubling selfishness afflicts city neighborhoods. To a crowd of 300 in the synagogue's basement meeting room, Mr. Koch conceded that joining in the city's program to care for thousands of homeless people in armories and city-run shelters could be troublesome for private groups such as churches and synagogues. Besides, he said, synagogues already do ''a lot of important social work.'' But none of it, he said, is the same as ''providing a bed.''

WHEN Angelo Rafael Donghia arrived in New York in 1959, the decorating world still basked in the light of the pink silk lamp shade. Elsie de Wolfe and Syrie Maugham were decorating deities. Billy Baldwin was king. Arrayed beneath them, a small, select hierarchy led by Sister Parish, Dorothy Draper and Eleanor Brown ordered swags for the small, rich hierarchy of New York, Los Angeles, Chicago and Palm Beach. The world had not yet been exposed to high tech or designer sheets. Although there were outposts of decorators in a few major American cities, the upholstered heart of that world was in Manhattan, where most young decorators wore navy blazers and did their own laundry. ''And almost no one made any money,'' said Mr. Donghia recently as he leaned back in a gray flannel chair in his gray flannel office. Profit, in those days, seemed oddly beside the point, if not impossible. Unless, that is, one happened to be Angelo Donghia. Like many other young decorators in town, his first job paid $75 a week.His apartment cost $60 a month. He had one good pair of tasseled loafers.

Two of the nation's largest banking organizations, the BankAmerica Corporation of San Francisco and the Continental Illinois Corporation of Chicago, yesterday reported sharp declines in fourth-quarter operating earnings. At BankAmerica, the nation's second-largest bank holding company after Citicorp, earnings before securities transactions dropped 13.2 percent, to $73.4 million, but the decline would have been much steeper if it had not been for a nonrecurring factor that bolstered the quarter's income by $30.8 million. Without that factor, the company's earnings would have declined by 50 percent, from the $84.6 million earned in the fourth quarter of 1981. On a per-share basis, operating earnings, before securities gains and losses, dropped to 46 cents, from 57 cents, the company said.

John S. Martin Jr. announced yesterday that he would resign as the United States Attorney in Manhattan on June 1. Mr. Martin, who plans to return to private law practice, will complete three years of a term that normally runs for four years with the approval of the President. He was appointed when Jimmy Carter was President and was retained when President Reagan took office. Asked if there had been any political pressure on him to resign now, the 47-year-old prosecutor replied, ''No, this is my decision, absolutely.'' Mr. Martin, a registered Democrat who has not been politically active, said he was resigning early to provide sufficient time for appointing a successor and assuring ''an orderly transition.''

President Reagan, cautioning that Japanese trade restrictions ''continue to weigh heavily'' on mutual relations, announced today that he and Prime Minister Yasuhiro Nakasone of Japan had agreed to press ahead with Japan's defensive rearmament and to study the issue of Japanese access to Alaskan oil. Mr. Nakasone, expressing sympathy for the effects of superior Japanese technology on the depressed American steel and auto industries, said Japan was reducing its trade and tariff barriers faster than any nation in the world. Describing a ''shower'' of protest at the White House and a ''storm'' of complaints on Capitol Hill, the Prime Minister cautioned the United States against taking protectionist countermeasures. ''I suggest that more positive, constructive efforts like encouraging Japanese industries to locate in the United States would be much wiser,'' Mr. Nakasone said at a news conference.

When Henry Doneger started his buying office business in 1946, he benefited from retailers' need for help in filling their depleted merchandise pipelines. In those postwar years, some 65 buying office companies emerged to advise and assist thousands of retailers in placing orders. In the 1950's and 1960's, the business thrived. However, in the past decade, as retail competition grew, multibillion-dollar chains developed and life toughened for independent stores. Many buying offices, pressed by the declining numbers of stores they represented, were caught in liquidations or mergers. And the number of buying offices now has fallen by more than half.

An article in Business Day yesterday incorrectly reported the percentage decline in Citicorp's net operating income for the fourth quarter. It was 12.4 percent.

''A high priority must be to get a hammerlock on this monster known as the Federal budget.'' - President Reagan. (A1:6.)

Agreement on a compromise to save the Social Security system was reached after a year of intense negotiations. Washington Talk, page B16. CHICAGO, Jan. 19 - President Reagan praised the recent agreement on revisions in the Social Security system tonight and called for further cooperation between Republicans and Democrats on the Federal budget this year. He also promised to present new proposals soon for job training for the unemployed.

China announced today that it was banning further purchases this year of cotton, soybeans and chemical fibers from the United States. The move was in retaliation for curbs imposed by the Reagan Administration on textile imports from China. The announcement, circulated by the official New China News Agency, also quoted Shen Jueren, director of the Foreign Trade Administration, as saying that China would reduce its planned imports of other American agricultural products, but he did not specify which ones. Restrictions on Textiles The ban comes four days after Washington imposed its restrictions because the two countries had failed to reach a new agreement governing China's textile sales to the United States. It also comes two weeks before Secretary of State George P. Shultz is to arrive in China for his first official visit.