What was going on when I was born?

Just about no one visited Saipan's velvety beaches this morning to recall D-day in the Pacific part of World War II. The 40th anniversary came and went, bold only in its contrasts with the emotions and speeches that flowed in Normandy on Europe's D-day, June 6. A few American veterans returned to this Mariana Island, certain that a commemoration would take place. Robert T. Kline of Paoli, Pa., came. So did Frank Moeller of Salt Lake City. Memories moved them. The lack of public acknowledgement disappointed them.

Mexico announced today that it had accepted an invitation from the United States and Nicaragua to play a role in talks between the two countries, but there was no confirmation from American officials. The move appeared to underscore the increasing role of Mexico, and to signal another shift in attitude by the United States, in the efforts to find a negotiated rather than a military solution to the Central American conflict. United States officials here and in Washington said they could not immediately confirm the announcement, which was made by the Mexican Foreign Ministry.

A police officer was shot to death with his own gun in a South Bronx store yesterday after he stopped a man accused of stealing a pair of eyeglass frames from a display case, the police said. The suspect was then shot twice by another police officer, who rushed into the store when she heard shots and saw the man standing, gun in hand, over the fallen officer, Juan Andino, the authorities said. Officer Andino, 39 years old, was pronounced dead on arrival at Lincoln Hospital. He had been shot twice, once behind the right ear and once in the left side of the head.

In what is seen as compelling evidence of an economic slowdown, the Government reported today that industrial production rose only four-tenths of 1 percent in May, the slimmest advance since November. It was the second smallest increase since the current expansion began at the end of 1982. Only the mining industry, aided by stepped-up production of oil, gas and coal, showed a major gain, according to the Federal Reserve Board's monthly tabulation of output from the nation's factories, mines and utilities.

Some of the city's best-known hospitals are facing periodic overcrowding in their obstetric units, ranging from a shortage of labor rooms to a lack of beds for women who have already delivered. This is happening, hospital officials say, in part because more women who live and work in the city are having babies and in part because many of these women are choosing to have their children at a select number of hospitals. It is also happening, they say, because of the unpredictability of the onset of labor itself and the economic pressure on hospitals to keep as many beds filled as possible. On some days, the obstetric units at such hospitals as New York Hospital, New York University Hospital, Presbyterian Hospital and Mount Sinai Medical Center have become so crowded that women have been asked to go through labor in the recovery room - spaces usually reserved for women who have already given birth.

Administration officials said today that the United States would resume limited economic talks with the Soviet Union and had also softened its opposition to talks with Moscow on curbing development of antisatellite weapons. The disclosure of these changes was interpreted by some officials as part of a general relaxation in tone and attitude toward the Soviet Union at a time of rising concern in Congress and throughout the United States and in Western Europe about sharpening tensions between Moscow and Washington. Administration officials said Mr. Reagan's offer on Thursday to ''meet and talk any time'' with the Soviet leadership also marked a small shift in attitude and tone. But the officials added that Mr. Reagan's aides remained skeptical of the chances for a summit meeting soon, or even of easing tensions.

Walter F. Mondale announced today that he would begin his search for a Vice-Presidential running mate by interviewing Mayor Dianne Feinstein of San Francisco and Senator Lloyd Bentsen of Texas. Mr. Mondale said ''additional conferences with other potential candidates,'' a process analogous to one used by Jimmy Carter in 1976, would be announced shortly. Aides said Mr. Mondale would probably announce further names on Saturday. Also, Mr. Mondale, who says he has enough convention delegates to win the Democratic Party's Presidential nomination, challenged President Reagan to at least half a dozen debates on specific issues, starting with arms control.

The Reagan Administration today refused to give Argentina any more time to qualify for a $300 million Treasury loan, thus pushing the difficulties over the third world's debts to a critical new threshold. The tone of both countries' negotiators today was conciliatory but their positions apparently intractable. Although the United States has left the door open for approval of the loan if Argentina meets its conditions, it indicated that Argentina must show some willingness to compromise. ''The political decisions that have to be made in Argentina don't appear ready to be made,'' said David C. Mulford, the Treasury's principal negotiator on developing countries' debts. ''We're ready to talk to them when they're ready.''

The anti-Sandinista military offensive begun in March in northern and central Nicaragua appears to have ended with rebel forces having established some bases inside the country, according to a Nicaraguan military commander. The Nicaraguan authorities also say they believe a force of 6,500 insurgents is regrouping and will begin a new and more destructive offensive in coming weeks. ''We know that it is coming and that we are going to have major confrontations,'' said Cmdr. Manuel Salvatierra, who oversees Sandinista operations in the area. ''We realize that this war is going to continue for a long time.''

Like Washington crossing the Delaware, Wilson Ferreira Aldunate, an exiled Uruguayan presidential candidate, plans to make his way across the choppy, chocolate Rio de la Plata to Montevideo to challenge the Government of Uruguay. This crossing, however, is political. A group of 500 supporters and journalists will accompany Mr. Ferreira Aldunate aboard a lumbering ferry, amid great international fanfare. The venture seems likely to end with Mr. Ferreira Aldunate's arrest.

A senior Iranian official said today that Iran was prepared to extend an agreement with Iraq against bombing civilian targets to cover attacks on shipping in the Persian Gulf, according to the official Islamic Republic News Agency. The Iranian News Agency said the official, Hojatolislam Hashemi Rafsanjani, the Speaker of the Iranian Parliament, was speaking in a sermon to a weekly prayer meeting at the University of Teheran. He welcomed the Iran- Iraq moratorium on attacks on each other's border cities arranged last weekend by the United Nations Secretary General, Javier Perez de Cuellar. Hojatolislam Rafsanjani then added, ''We say the same thing about the Persian Gulf.''

The Liberal-Social Democratic alliance, which has languished in the political doldrums since the Falkland War gave a lift to Prime Minister Margaret Thatcher and her Conservative Party, came back to life today with an upset victory in a key by- election. Michael Hancock, a 39-year-old Social Democrat, captured the constituency of Portsmouth South against the predictions of the opinion polls and the major party leaders.