What was going on when I was born?

Walter F. Mondale accused President Reagan today of an ''inexcusable'' attempt to shift the blame for last week's bombing of the American Embassy near East Beirut. Earlier, the White House sought to soften Mr. Reagan's implication that the fault lay with the ''near destruction of our intelligence capability'' before his Administration took office. At a news conference after his meeting in New York City with Andrei A. Gromyko, the Soviet Foreign Minister, Mr. Mondale asserted that Mr. Reagan should quit trying to pass on the blame for the incident. ''The latest statement by the President is inexcusable,'' Mr. Mondale said. ''He should stand up and say he is responsible. By saying the C.I.A. is weak, he encourages terrorists and our enemies around the world to believe that we don't have an effective intellignce capacity, when we do.''

NEW YORK is the only city in America, and one of the few in the world, with two major opera companies. Yet for reasons of scheduling, tonight, tomorrow and Sunday mark the first weekend in nearly a year that the Metropolitan Opera and the New York City Opera are offering simultaneous performances. In the past, the Met ran from September to April - it still does - and the City Opera gave two half-seasons, one in the fall and one in the spring. But in 1982, Beverly Sills, the general director of the City Opera, announced her intention to consolidate the company's two seasons into one summer-fall season, running from early July until mid-November. Her plans were frustrated the first year, in 1983, by an orchestra strike, although the City Opera and the Met did perform together last fall. This year, the City Opera has been operating at full tilt since July, but the Met only swung back into action Monday, with Placido Domingo, Anna Tomowa-Sintow and Eva Marton in Wagner's "Lohengrin."

THE unprepared visitor who walks into the Museum of Modern Art any time between now and Jan. 5 may be startled to find that a large part of the museum has been taken away from modern art. In its place, we find primarily a community of vagabond spirits that originated quite some time ago in Africa, Oceania and the Americas. The spirits in question inhabit the standing figures, the reliquary figures, the fetishes, the headdresses and, above all, the masks that fill room after room. Irrespective of their date, place of origin or primary function, these objects have a presence, a fierce contained energy and a robust conceptual approach to the human form. They were not conceived as likenesses of something else, but as themselves. For the most part, they are shown in a subdued light, as if what they have to say to us were confidential, and when we look at them, we are at once thrilled and disoriented, awed and a little alarmed. Such eyes, such noses, such a superabundance of hair and such chain-saw teeth have rarely come our way.

At exactly 8:30 Friday morning, the Government will publish what might seem to be its most important economic statistic: the figure designed to predict the course of the $3.6 trillion-a-year American economy. Whether the 21-month old economic expansion is slowing down, stalling out or perhaps even slipping into recession is of overwhelming interest to business planners, stock and bond operators, consumers, workers and economists, not to mention politicians with eyes fixed on Election Day. The leading index for August is expected by many economists to eke out a small gain, mainly the result of a very strong stock market last month. But it could also fall slightly, thereby producing a third consecutive decline, which is popularly regarded as a sign of impending recession.

Sweeping organizational changes were announced yesterday by AT& T Information Systems, the company formed to market computers, phone switching systems and other advanced office equipment for the deregulated American Telephone and Telegraph Company. The reorganization, the second in two years, came amid reports that the company has been unable to fashion a winning market strategy. In an announcement from its headquarters in Morristown, N.J., AT& T Information Systems said it would be divided into three divisions - one to serve large corporations, a second to serve small and mid-sized concerns as well as consumers, and a third to manufacture computers that it would then sell to retailers or supply to the two other groups. The key to the reorganization, the company said, is that each of the new divisions will have capabilities in research, manufacturing and marketing. Until now, AT& T Information Systems has been mainly a marketing arm, with development and assembly of products kept separate.

Gov. James B. Hunt Jr. said today that he would not block the execution of Margie Velma Barfield, a 51-year-old woman convicted of killing her fiance with arsenic. Mrs. Barfield's execution is scheduled for Nov. 2, four days before the election in which Mr. Hunt, a Democrat, seeks to oust Jesse Helms, a Republican, from the Senate, and the issue has figured in the political contest. Mrs. Barfield would be the first woman executed in the United States in 22 years. Under state law, she may choose to be executed by gas or by injection of lethal drugs.

Friday BACH, UPTOWN AND DOWN Anticipating Bach's 300th birthday next March 21, the Cathedral of St. John the Divine at 112th Street and Amsterdam Avenue is offering free concerts of his works tonight, Saturday, Monday and Tuesday at 7:30. Tonight's performers will be the St. Luke's Chamber Orchestra, conducted by Michael Feldman, with Caroline Stoessinger, piano; D'Anna Fortunato, soparano; Dora Schwarzberg, violin, and Stephen Taylor, oboe. On the program are the Fifth Brandenburg Concerto, the Keyboard Concerto in D minor, the Wedding Cantata and the Double Concerto for oboe and violin. As part of its own series, St. Luke's will also play all six Brandenburg Concertos on Saturday at 8 P.M. at St. Joseph's Church, Waverly Place and Avenue of the Americas. Tickets: $10; reservations: 226-1115. BALANCHINE FOR TV Dances that George Balanchine adapted, and in one case created, for television, are being shown through Nov. 15 at the Museum of Broadcasting, 1 East 53d Street. Each day, Tuesdays through Saturdays, there will be 90-minute screenings at 12:30, 2:05 and 3:40 P.M., and, on Tuesdays, 6 P.M. as well. Today and tomorrow the package consists of two broadcasts for Canadian television. One features ''Liebeslieder Walzer'' (1961), danced by four couples to music of Brahms, the other, ''Who Cares'' (1971), performed by 24 dancers to Gershwin songs. Suggested contribution: $3 ($2 for students, $1.50 for over-65's and under-13's). Information: 752-7684.

While waiting to clear last- minute legal hurdles, New York State is preparing to close the 1,000-bed state prison here on Monday. The closing of the seven-story prison, which underwent a $40 million rehabilitation two years ago, will force the state to make space for the 513 remaining inmates in a correction system already burdened by severe overcrowding. Unlike previous closings, the prison - the former Pilgrim State Psychiatric Center - is not being shut because it is uninhabitable. Instead, the action fulfills a pledge by Governor Cuomo in the heat of a bitter gubernatorial campaign two years ago.

The Market Place column in Business Day Monday incorrectly reported the employer of Jeffrey Atkin, a securities analyst. It is Cable, Howse & Ragen of Seattle.

The Economy The prime lending rate was cut to 12 3/4 percent, from 13 percent, by the nation's major banks. Bankers cited a general decline in interest rates and a slow demand for loans, but they hesitated to predict further declines. The move comes nearly a week after Morgan Guaranty trimmed its rate to the same level, and a day after Wells Fargo cut its prime to 12 1/2 percent. (Page D1.) Coal miners appear to have voted overwhelmingly in favor of a new contract. (A16.)

A CONTEMPORARY American story launches this year's New York Film Festival, which begins tonight at Lincoln Center's Alice Tully Hall and runs through Oct. 14. The opening movie, ''Country,'' stars Jessica Lange and Sam Shepard as Iowa farmers who are battling a hostile government bureaucracy willing to foreclose their farm loans in exchange for a favorable bottom line - even if that means destroying the farmers' lives. Directed by Richard Pearce, the film (review on page C20) lends an uncharacteristically home-grown flavor to the premiere of a festival that has long been more associated with foreign films than American. This year, 9 of the festival's 26 movies are American, and 6 are American independent films - more than have ever been included. Moreover, this year's closing film provides a certain kind of symmetry. ''Paris, Texas'' was written by Mr. Shepard, the Pulitzer Prize-winning playwright, from a short sketch about a man whose wife deserts him; unable to face raising their son alone, he deposits the child with his brother and sister-in- law and disappears. Directed by Wim Wenders, the film stars Nastassja Kinski and Harry Dean Stanton and is set in Texas.

For six years, the New York City Police Department was the round- the-clock, relentlessly high-tension preoccupation of Robert J. McGuire. As commissioner, he oversaw thousands of employees, tried to manage a succession of life-and- death crises, and did what he could to satisfy often antagonistic community groups. Today, as chairman and chief executive officer of Pinkerton's Inc., the largest and oldest security company in the world, he still oversees thousands of employees, but there the similarity ends. His empire extends to dozens of offices around the world now, and he spends most of his time planning corporate acquisitions and marketing strategy. Also, he has relaxed a bit.