What was going on when I was born?

Mayor Koch yesterday vetoed legislation he had proposed that would have banned food peddlers from most of midtown Manhattan. He said he had decided the bill was too restrictive. It is only the third time that Mr. Koch has vetoed a bill, and it was the first time he killed a bill that the City Council had approved at his request. He did so, he said, because he decided he had been wrong.

International South Africa's police are holding 300 people after carrying out more predawn raids, according to civil rights monitoring groups. They said that priests, lawyers, teachers and political activists were among those who have been detained under the country's newly imposed state of emergency. [Page A1, Column 5.] Apartheid is largely responsible for the violence in South Africa, the Reagan Administration asserted. The statement was sharper in tone than earlier expressions of concern over increasing unrest in South Africa's black townships. [A1:6.]

These have been months of political siege for Defense Secretary Caspar W. Weinberger. Since February, he has been battled to a standstill on the military budget and the MX missile. His influence on some national security issues, such as arms control, seems at least somewhat diminished, and his credibility is under attack on Capitol Hill. Some conservatives charge that he has let the world's largest bureaucracy run out of control and has failed to set priorities for spending and strategy. Dramatic Shifts Not Expected In private, Administration colleagues speculate whether Mr. Weinberger, one of the last survivors of President Reagan's original Cabinet, will endure at the Pentagon, where only two secretaries have had longer tenure.

THIRTY schoolteachers who normally stand in dignified positions at the head of their classes were scattered about a dance rehearsal hall at the Juilliard School at Lincoln Center one day recently, struggling to lend some grace to the leaps and turns they were attempting. Dressed in shorts, sweatpants and T-shirts, they puffed their way across the floor, trying to imitate the choreography in ''Aureole,'' a dance by Paul Taylor that they had seen performed the previous day. Some of the more lithe teachers might have been mistaken for dancers who never made it, but most, overweight and out of shape, were not likely to be taken for anything but what they were: middle-aged nondancers trying to understand the movements of the dance. The experience was made possible by the Lincoln Center Institute, a program that enables teachers in elementary and secondary schools in the metropolitan area to deepen their knowledge of the performing arts under the tutelage of dancers, actors and musicians.

The White House offered an Indian peace pipe today to Senator Bob Dole and said President Reagan would meet with Congressional leaders and make telephone calls to help promote a budget compromise. But in answer to Mr. Dole's request for Presidential assistance in breaking an impasse between the House and the Senate, the officials said Mr. Reagan would not take the lead. Mr. Dole, the Senate majority leader, asked the President on Sunday to ''step into the breach.'' ''I don't think he's going to ride a white horse to Capitol Hill on this one,'' Larry Speakes, the White House spokesman, said this morning. ''But he'll be there and his presence will be forceful.''

BIOLOGISTS studying garter snakes in Canada have discovered that some males mimic females in a manner never before seen in the animal world. Exuding female-like chemical scents, the mimics entice the courtship of other males. By sowing confusion among their male rivals in this manner, the mimics improve their own chances for success in the desperate competition for bona fide female mates. In their effort to understand the driving forces of evolution, biologists are paying increased attention to the diversity of methods by which individuals within a species compete for sexual partners. Male-male combat, for example, is the common way that males of many species gain preferential access to females. But female mimicry and other tactics of deception are among the recently discovered alternatives to aggression in the never-ending struggle of animals to reproduce.

The Reagan Administration said today that apartheid was ''largely responsible for the violence'' in South Africa. The White House spokesman, Larry Speakes, offered the Administration's views at a White House briefing this morning, and they were later echoed by State Department spokesmen. The remarks went beyond an Administration statement Saturday that it was ''deeply troubled'' by the continuing unrest in South Africa. The violence was cited by President P. W. Botha on Saturday in declaring a state of emergency in several areas of South Africa.

The South African police, acting under a newly imposed state of emergency, were reported to have detained scores more people in predawn raids today. Civil rights monitoring groups said about 200 people were seized in the last 24 hours and put the number arrested since the emergency declaration took effect midnight Saturday at more than 300. Priests, lawyers, teachers and political activists were said to be among those detained last night and today. They joined 113 people seized Sunday by the police, who now have near-absolute powers in 36 cities and towns covered by the emergency decree issued by President P. W. Botha. The police made no immediate comment on the reports.

An Israeli court today sentenced three Israeli Jews convicted of terrorism to life in prison and 12 others to terms ranging from 3 to 10 years. The 15 were convicted July 10 by a three-judge panel of several crimes against Palestinian Arabs in the West Bank. The acts included murder, belonging to a terrorist organization, manslaughter, maiming two Arab mayors and plotting to blow up one of Islam's holiest shrines, the Dome of the Rock, on Temple Mount in Jerusalem. Under Israeli law, life terms are mandatory in murder convictions. All the nonmurder sentences were partly suspended, reducing to seven years the next longest term. The sentences were widely seen by Israelis as relatively lenient, considering the crimes.

PATIENTS who are dissatisfied with their psychotherapists are probably better off switching to someone else, according to new research that challenges conventional thinking in the field. While few would disagree that a patient who is dissatisfied with a therapist during the first few meetings should switch, the new view is that a switch may well be for the best weeks or even months into treatment if dissatisfaction appears and persists. This thinking is grappling with one of the most difficult areas of psychotherapy, the notion of resistance. In psychoanalytic therapy expressions of dissatisfaction -or resistance - are seen as an expected, even beneficial, step in the therapy's progress. Other therapists who are less classically oriented than the analysts have absorbed this attitude, too, tending to see patients' dissatisfaction as a normal expression of their problems. All of this makes the seemingly simple decision about whether a patient has found a suitable therapist a knotty one in which therapists may discount what may be justified dissatisfaction as yet another symptom. Patients may be led to dismiss the dissatisfaction as well.

A front-page picture caption yesterday with an article about the mayoral race in New York City misstated the time of Carol Bellamy's campaign appearance in Central Park. It was Saturday night.

The New Jersey Supreme Court today caught up with the ''man with the golden arm,'' Nicholas Caputo, the Essex County Clerk and a Democrat who has conducted drawings for decades that have given Democrats the top ballot line in the county 40 out of 41 times. Mary V. Mochary, the Republican Senate candidate, and county Republican officials filed a suit after Mr. Caputo pulled the Democrats' name again last year. The election is over - Mrs. Mochary lost -and the point is moot. But the court noted that the chances of picking the same name 40 out of 41 times were less than 1 in 50 billion. It said that ''confronted with these odds, few persons of reason will accept the explanation of blind chance.''