What was going on when I was born?

Sales of new cars by the nation's major manufacturers rose 36.6 percent in the last 10 days of April from a year earlier, the companies reported today. The increase was measured on a daily rate basis, with eight selling days in the period this year and nine last year. The seasonally adjusted annual selling rate, which has been bouncing up and down the last two months, rose to 8.3 million in the April 21-30 period, from 7.7 million in mid-April, to reach 8.1 million for the month, about what analysts had expected. Ann Knight, an auto analyst with Paine Webber Mitchell Hutchins, said: ''These are good strong sales. They are sustainable. When we are in a period resembling normal demand, as we are now, you're going to see some fluctuation from one 10-day period to the next. Between now and the end of the second quarter, anything between a 7.5-million and 8.5-million rate will not surprise me.''

Esmark Inc., the big Chicago company that bought Norton Simon Inc. last year for $1 billion, said yesterday that it had just received a takeover bid of its own for $2.3 billion in cash. It has yet to respond to the offer, but Esmark is widely known to be interested in making a deal, if an acceptable price can be struck. In a brief statement announcing the deal, Esmark said that it had been offered $55 a share for the company's 41.8 million shares outstanding, or $2.3 billion, by Kohlberg, Kravis, Roberts & Company, a New York investment house that specializes in management buyouts. Wall Street analysts, forecasting a bonanza for Esmark stockholders, said they expected other suitors to join in the bidding soon.

An obituary on April 15 of Carl K. Erpf, chairman of the Manhattan brokerage firm of Ladenburg, Thalmann & Company, misidentified Mr. Erpf's late father. He was Carl Erpf; Armand G. Erpf was the younger Mr. Erpf's uncle.

Because of a mechanical error, some copies of the Late Edition yesterday omitted page C11 and carried page C18 twice. Readers wishing copies of the missing page may write to Production Quality Control, The New York Times, 229 West 43d Street, New York, N.Y. 10036.

The Internal Revenue Service is proposing to ban a popular new type of fringe benefit plan that allows workers to use pretax wages to pay for medical and other personal expenses and take back in cash any unused portion. The regulations would require hundreds of thousands of workers who participated in one type of these so- called reimbursement or flexible spending accounts to pay back taxes and interest on benefits previously received. The reimbursement accounts are an outgrowth of the so-called cafeteria approach to employee benefits, whereby participants may choose from a menu offering day care, legal and medical and dental coverage. In all, employee benefit consultants estimate that as many as 200 major reimbursement plans covering between one million and three million employees will be affected by the regulations. Those who participated in the most liberal of these plans, known as zero-balance reimbursement accounts, or Zebras, will likely have to pay interest and taxes for any amounts reimbursed in the last three years, an I.R.S. spokesman said.

A commentary distributed by Tass, the Soviet Government press agency, accused China today of taking only scant notice of the United States' ''militarist course'' while seeking improved relations with Washington. The unsigned assessment of President Reagan's visit reiterated previous Soviet press reports that Mr. Reagan had given the visit a ''provocative anti- Soviet orientation'' and had sought ''to play the 'China card' '' against Moscow. But diplomats noted that the commentary today was also critical of the Chinese leaders, charging them with having tacitly or even openly supported Mr. Reagan's policies. Deng Said to Back U.S. Buildup The commentary said Deng Xiaoping, the paramount Chinese leader, was supporting the United States arms buildup, and it said the Chinese either took no notice or issued only minimal criticism of American policies toward the Soviet Union, Asia, Latin America and even Taiwan and Communism.

An article on Wednesday about allegations that gangs had forced tenants out of some Manhattan apartment buildings included a list, provided by the District Attorney, that cited one building erroneously. The building, 336 West 19th Street, should not have been on the list.

President Reagan and Congressional leaders of both major parties defended the American Ambassador to El Salvador today against sharp criticisms made Wednesday by Senator Jesse Helms. In a letter to the President that was originally made public in El Salvador, Senator Helms accused the Ambassador, Thomas R. Pickering, of interfering in the Salvadoran presidential runoff election and demanded the envoy's immediate recall. Larry Speakes, the White House spokesman, said Mr. Reagan had ''full confidence'' in his Ambassador and had no intention of removing him. Moreover, Mr. Speakes said, the Administration ''is taking no sides'' in the election Sunday, which pits a moderate leftist, Jose Napoleon Duarte, against an outspoken conservative, Roberto d'Aubuisson.

The Mark Rothko Foundation yesterday announced that it was giving most of its collection of about 1,000 works by Rothko, one of the leading artists of the 20th century, to 19 museums in the United States and abroad. The National Gallery of Art in Washington will get what the foundation described as the core of the collection - 285 paintings and works on paper, as well as 500 to 600 sketches, drawings and other study materials. The gift of the works entrusted to the foundation brings to a close a long, sometimes sordid and often painful chapter in art history. Suit Begun in 1971 Rothko committed suicide in 1970; in his will he left almost all his works to the foundation, which he had established a year earlier. In 1971, Rothko's children accused the three executors of their father's estate - men who were his close associates and members of the foundation - of waste and fraud in transferring several hundred paintings to the Marlborough Gallery, where they were sold at terms below their true value.

''There is no substance to the charge that the Ambassador has sought to 'rig the outcome' of the elections, or to 'manipulate the electoral process.'

Alan Schneider, one of the most important American directors of contemporary theater, died yesterday in London from head injuries suffered when he was hit by a motorcycle Monday. He was 66 years old. Mr. Schneider, who was in Britain to direct a play, never regained consciousness after the accident. During a pioneering and prolific career that lasted 40 years, Alan Schneider became the primary American director of Samuel Beckett's plays, and also a notable director of the works of Edward Albee, Harold Pinter, Michael Weller, Bertolt Brecht and many other playwrights. Any list of productions of the most significant plays of the mid-20th century would often have to read, ''Directed by Alan Schneider.''

Secretary of Defense Caspar W. Weinberger today gave Congress the Reagan Administration's plan to cut the military budget. It bows to bipartisan pressure to shrink the Federal deficit but saves the scope of the military buildup by stretching out purchases of conventional weapon programs. The new cuts decreased the President's requested Pentagon appropriation for the fiscal year 1985 by $13.9 billion, to $291.1 billion from $305 billion. This would bring 7.8 percent growth in the military budget next year after making up for the effects of inflation.