What was going on when I was born?

A Philippine athlete who vanished Dec. 29 after secretly marrying a daughter of President Ferdinand E. Marcos apologized on radio and television today because his family had accused the Marcos family of complicity in his disappearance. Tommy Manotoc, a 32-year-old amateur golfer and professional basketball coach, made the apology by twice reading a letter to the President at a news conference arranged by Defense Minister Juan Ponce Enrile. The news conference was held hours after the Government announced that he was rescued from ''armed terrorists'' Monday in a mountainous area 55 miles east of Manila ''usually dominated'' by the Maoist insurgent group known as the New People's Army. ''I understand,'' Mr. Manotoc's letter said, ''that many names have been maligned and that your honor has been questioned. This I would want to rectify for the sake of the country and the Filipino people which you lead.''

Israel has reportedly been considering plans for a large-scale invasion of southern Lebanon to clear out the Palestinian military reinforcements that have been brought into the region since the cease-fire there last July. The Government has also reportedly told the United States that an attack would be launched should there be further terrorism by the Palestine Liberation Organization. Troops and weapons of the Israeli Army are said to be deployed in the north to carry out such an operation, if necessary, according to foreign diplomats here. Both Israelis and foreigners familiar with the high-level deliberations say no final decision has been made to go ahead with an invasion. A proposal to do so last week by Defense Minister Ariel Sharon, after a five-man terrorist squad crossed into Israel from Jordan, was rejected by Prime Minister Menachem Begin because of the unfavorable repercussions it would have on relations with Washington, officials say.

No one worries about sticker shock in George Principe's wood-paneled Cadillac showroom. While other dealers were fretting last year about the recession and slumping sales and buyers' preference for foreign cars, he and his six salesmen sold 996 new Cadillacs - more than ever before. But the record sales have not completely protected Mr. Principe from the problems that have troubled Cadillac's poorer cousins within the General Motors family. The dealership's profits were held down by rising operating costs, including wage increases that followed a weeklong mechanics' strike last year.

The Koch administration, which discontinued payroll deductions for United Way of Tri-State more than a year ago because of reservations about the way they were conducted, announced yesterday that it had devised a new plan. The Mayor's office said the new procedure had been designed to remedy several problems he had with the old plan, which was administered by United Way. The city let that system lapse in late 1980. At the time, Mr. Koch questioned United Way's methods of soliciting money from city workers and noted that, through an apparent administrative error, some weekly deductions had been continued longer than they should have been.

The State Public Service Commission agreed today to allow the completion of the Nine Mile Point 2 nuclear power plant on the shore of Lake Ontario, despite overruns that have caused an increase of more than tenfold in cost projections in eight years. At the same time, the chairman of the commission said it had reached a consensus on setting up a system of incentives and penalties that would force the stockholders of the five utilities building the plant to bear some of the cost of future overruns. The Commission staff estimated that the plant would cost $4.9 billion when completed in 1987, making it the most expensive nuclear power plant ever in terms of the construction cost per kilowatt of power. In 1974, when construction began, the cost was projected at $381.7 million.

United States interest rates have become the driving force behind exchange rates, replacing such more fundamental factors as balance-of-payments deficits and inflation, according to a variety of experts. In currency markets around the globe, movements in interest rates on Treasury bills and other American investment instruments have come to receive close attention. Even the weekly announcement of money supply growth by the Federal Reserve enjoys almost as much scrutiny from foreign exchange dealers as from credit market specialists. ''You can see it in world financial markets ,'' said Rimmer de Vries, senior vice president of the Morgan Gu aranty Trust Company. Referring to his bank's monthly publication, he noted that ''three ofthe last five issues have been all about the United States.''

Consumer installment debt fell $173 million in December, as Americans paid off more than they borrowed for the first time since July 1980, another recession period, the Federal Reserve Board reported today. The big decline in buying on credit -including an expected new drop in auto loans - was further evidence that the current recession was worsening as 1981 ended. Earlier reports had shown sluggish sales, quickly falling production and rapidly rising unemployment during that month.

Despite the reservations of several of its members, the Landmarks Preservation Commission yesterday designated a former synagogue on the Upper West Side as a landmark. The designation of the vacant building, at 130 West 79th Street, formerly the Mount Neboh Synagogue, came at a daylong hearing of the commission at City Hall where possible designation was discussed for several better-known buildings. Included among these were the Woolworth Building, at 233 Broadway; Lever House, at 390 Park Avenue; the Hearst Magazine Building, at 959 Eighth Avenue; the B. Altman & Company building, at 355 Fifth Avenue, and Pomander Walk, an enclosed, 63-unit apartment complex between West 94th and 95th Streets and West End Avenue and Broadway that was designed to resemble a London street. Votes Are Put Off Decisions on those properties and nine others were put off for later hearings. Most of those who spoke argued for landmark designation - several supporters spoke for more than one building - but representatives of the owners of many of the properties requested that votes not be taken.

President Reagan's proposed budget for the next fiscal year was assailed by both Republicans and Democrats on the House Appropriations Committee today as the Administration's top economic officials began formally presenting the budget to Congress. At the same time, Senate Republicans, stunned by the projected $91.5 billion deficit, indicated that they would rewrite the budget on Capitol Hill. As a measure of his concern, the Senate Republican leader, Howard H. Baker Jr., expressed interest in a Democratic proposal for a partial freeze on Federal spending and tax cuts. Arguing the case for the President's $757.6 billion budget before the House Appropriations Committee, Treasury Secretary Donald T. Regan, Budget Director David A. Stockman and Murray L. Weidenbaum, chairman of the President's Council of Economic Advisers, found themselves whipsawed. 'Can't Agree on Priorities'

NO other city in the United States and possibly in the world offers its food shoppers easy access to such a wide variety of fruits and vegetables as does New York. On virtually every other street corner, it seems, shops display glistening rows of vegetables and shiny mounds of fruit all day long seven days a week. Because a lot of the produce looks wonderful, we tend to assume it is fresh. Is it really? The answer depends on how ''fresh'' is defined. Like other words used to describe food, freshness is relative. If it suggests produce just out of the field or off the tree, New York has none at this time of year and very little even in This is the first of four articles on freshness in food. the growing season. But ''fresh,'' according to Webster's New World Dictionary of the American Language, means recently produced or grown, having original quality and taste, and especially not salted, preserved or pickled and not spoiled, rotten or stale.

Consumer installment debt fell $173 million in December as Americans paid off more than they borrowed for the first time since July 1980, the Federal Reserve Board reported. It said credit extended for new purchases declined 2.3 percent to a seasonally adjusted $26.87 billion. The figure was seen as new evidence that t herecession was worsening as 1981 ended. (Page D1.) President Reagan's budget request was attacked by members of both parties as Administration officials began their formal presentation of the proposal to Congress. Senate Republicans indicated they would rewrite the budget, voicing interest in a Democratic plan to place a partial freeze on Federal spending and tax cuts. (A1.) Mr. Reagan, continuing his Middle Western swing, used some of the harshest language of his Presidency in defending his budget. (A1.)

Deputy Commerce Secretary Lionel H. Olmer told Congress today that, despite strong objections from its European allies, the United States believes it can enforce its export control laws overseas to block the use of American technology in constructing a proposed 3,000-mile pipeline from Siberia to Western Europe. Mr. Olmer, along with Ernest B. Johnston Jr., Deputy Assistant Secretary of State, and Stephen D. Bryen, Deputy Assistant Secretary of Defense, told a House subcommittee on science and technology that the Administration was increasing pressure on the Europeans to withhold their support for the pipeline, which would bring them natural gas later in the decade. The Europeans have already contracted to provide financial backing and equipment for the $10 billion construction project. It would provide employment and new business for their sagging economies and diversify their energy supplies.