What was going on when I was born?

The man who is believed to be the leader of Omega 7, the anti-Castro terrorist group, was arrested in Miami yesterday afternoon. According to Federal authorities, Omega 7 has been one of the most active terrorist groups in the United States. It has taken responsibility for at least two murders and more than 30 bombings in Miami, New York, New Jersey and Washington in the last seven years. The suspect, Eduardo Arocena, was charged with plotting to assassinate the Cuban Ambassador to the United Nations in New York more than three years ago.

The International Airport here came under rocket fire today and heavy artillery duels followed between Lebanese Christian and Moslem Druse militia forces in the nearby mountains. At least 21 people were reported killed and 45 wounded. Three Americans - two marines and a Navy enlisted man - were among the wounded at the airport. Most of the casualties were in Christian districts. The toll in Druse areas was not immediately known. Nor was it definitely established at once who fired at the airport, although the Lebanese Army said the rockets - 15 Sovietbuilt surface-to-surface missiles - had come from positions in Shuweifat, a Druse town four miles to the southeast.

Rita M. Lavelle, a former Federal environmental official, today was found not guilty of contempt of Congress after two days of testimony about her failure to appear at a Congressional subcommittee hearing. Miss Lavelle, smiling, said afterward that the verdict ''strengthens my faith in the system, the American system, that it will work.'' Her dismissal as head of the Environmental Protection Agency's toxic waste disposal program Feb. 7 touched off a storm of charges of wrongdoing aimed at Miss Lavelle and others at the agency. Verdict Cannot Be Appealed The jury verdict in Federal District Court today ends the Government's contempt-of-Congress case against Miss Lavelle. The prosecution cannot appeal a jury's verdict in a criminal case because of the Constitution's protection against double jeopardy. However, the Justice Department is apparently still looking into allegations that she may have committed perjury in Congressional testimony on other occasions.

A police helicopter and a commuter seaplane collided over Brooklyn yesterday morning, sending the helicopter into a vacant building below and the plane into the choppy water of New York Harbor. The two police officers in the helicopter were killed, and two men were trapped in the seaplane when it sank. Moments before the seaplane went under, two passengers escaped into the water. One swam to shore and the other was pulled out by tugboat crewmen who dived into the water to make the rescue, trying in vain to keep the plane afloat by tying ropes to it. Tail Rotor Torn Off The helicopter's tail rotor was torn off by the the collision. The blue craft flipped through the air, hit a three-story brick building, scraped down the building's side and crashed through the roof of a vacant garage. One witness said that one of the policemen tried to jump out and grab the roof of the building but could not hold on.

President Reagan has approved a plan calling for a substantial increase in American military involvement in Central America and preparations for a possible limited military blockade of Nicaragua, senior Administration officials said today. Military operations that the United States will soon conduct in Central America and the Caribbean are designed to lay the groundwork for the expanded American presence and the possible quarantine, the officials said. The series of air, sea and land operations, which are scheduled to begin next month, are to last a minimum of six months. In public comments officials have described the exercises as routine.

President Reagan announced today that Philip C. Habib was stepping down as his chief Middle East negotiator and would be replaced by Mr. Reagan's deputy national security adviser, Robert C. McFarlane. The action was described by a White House official as part of a new effort to end the impasse over bringing about the withdrawal of foreign troops from Lebanon. The President said Mr. Habib was leaving because of private commitments, but a senior State Department official told reporters that a major reason for the change was Syria's refusal to receive Mr. Habib as a negotiator in recent months. The unexpected move was announced by Mr. Reagan as he said farewell at the White House to President Amin Gemayel of Lebanon. The Lebanese leader has spent several days here trying to press the Administration into doing more to persuade Syria to agree to a formula for pulling its troops out of Lebanon in tandem with Israel, which has already accepted an accord for doing so.

Consumer prices rose only two-tenths of 1 percent in June and only 2.6 percent for the last 12 months, the Government reported today. The 12-month rate was the lowest in 15 years, a fresh, strong sign that the high inflation that plagued the economy in recent years has been checked. The Labor Department's report on the Consumer Price Index for June showed that the rising gasoline prices that caused a spurt in the index in April and May had stabilized, while food prices tumbled. As the recovery continues, mainstream economists say that lingering inflationary pressures could propel the rate of price increases a percentage point or two above the recent, subdued pace - but no more than that.

The United States Embassy in Honduras, once a Foreign Service backwater, has over the last two years become the largest and most important American mission in Central America. To Hondurans and foreign diplomats in this capital, the embassy - two large and well-fortified buildings facing each other across one of Tegucigalpa's main thoroughfares - has become the symbol of the rapidly growing American presence in Honduras, the country that is now the focal point of Washington's policy in the region. It is from Honduras, Western diplomats and Honduran opposition politicians say, that the Reagan Administration is waging a psychological, economic and military campaign against the Sandinista Government of neighboring Nicaragua. By the embassy's own count, the United States now has close to 300 military advisers, technicians and engineers in Honduras, not counting the eight officers and nine enlisted men attached to the military attache's office and the Military Assistance Group. The embassy staff numbers over 140, at least 35 of them with diplomatic rank. Most other foreign missions here, according to American diplomats, have fewer than six foreign service officers. Washington also has a strong and experienced Ambassador, John D. Negroponte, who is often rumored to be under consideration for higher policy-making posts in the Reagan Administration.

The pressures of literary fashion are very mild here: no foreign books have been legally imported since the military Government decided two decades ago that they served no useful purpose. ''Ah, Maurice Collis,'' said a gray-haired Burmese to a browser at a bookstall here. ''I've read him for years. A British civil servant. 'Trials in Burma.' 'The Journey Outward,' et cetera.''

The Egyptian Ambassador to the United States, Ashraf Ghorbal, said today that his country's failure to make a $30.2 million interest payment on debts for purchases of United States weapons was an oversight. The Ambassador said a payment missed for June and another due July 15 ''will be paid - there is no question.'' Mr. Ghorbal, asked about the issue at an embassy reception, said recent Moslem religious fasting and subsequent feast days were behind Cairo's failure to pay the June 15 interest installment on a $2.1 billion debt.

Two leading American foundations, in an effort to help Zimbabwe make its education system more responsive to the needs of its predominantly black population, have provided a $1 million grant to the country's only university. The grant is being made jointly by the Ford and Rockefeller Foundations.

Robert C. McFarlane, who was named by President Reagan today to succeed Philip C. Habib as chief Middle East negotiator, has won praise from so many quarters that many people reacted with dismay when they learned he would be moving on to another post. In his job as deputy national security adviser, for example, Mr. McFarlane - a veteran of diplomatic and bureaucratic battles in three Republican administrations - is viewed inside and outside the White House as the indispensable negotiator with Congress, as in Mr. Reagan's recent efforts to get money approved for the MX missile. ''More than anyone else in this Administration, Bud McFarlane is the guy in this Administration who has had the confidence of the members of Congress,'' said Representative Les Aspin, Democrat of Wisconsin. ''He's the one guy who has been totally in command of the technical issues and the political nuances.''