What was going on when I was born?

The spring collections peaked here with the showing Tuesday night of the Giorgio Armani collection in the theater of his headquarters on the Via Durini. Not a seam was out of place as the master tailor brought out his now-classic daytime look based on a precision-made broad-shouldered jacket. But there was a surprise. Using spangled fabrics that glittered as they caught the light and some prints, he reinterpreted his jackets and shirts for evening. His evening separates were applauded last season; this year they overshadowed the daytime clothes. There are no tricks and no shockers, just a lot of pretty clothes that put a new slant on night dressing. Sufficiently formal for almost any occasion, they appeal especially to women who do not care for traditional evening dresses with their bare necklines and draped body lines. Armani's jackets and skirts, no matter how glittery, preserve the casual feeling of his daytime clothes. For many women, they will offer a comfortable way to dress up at night.

The United States tonight assailed ''the brutal killing'' of an American aboard the hijacked Italian cruise ship, and it criticized Italy and Egypt for failing so far to announce that the hijackers would be prosecuted. State Department officials said they were furious at the speed with which the Italians and the Egyptians, working with the Palestine Liberation Organization, agreed to let the hijackers go free. A statement read by Larry Speakes, the White House spokesman, said, ''We are saddened and outraged at this brutal killing'' of Leon Klinghoffer of Manhattan, a passenger aboard the hijacked ship, the Achille Lauro. Confirmation by Ambassador Confirmation of his death came from the United States Ambassador to Egypt, Nicholas A. Veliotes, who was sent today by President Reagan to make a first-hand inspection of the cruise ship several hours after Egyptian authorities announced that the hijacking crisis had ended.

Four men identified as the hijackers of an Italian cruise ship surrendered in Egypt on Wednesday after a two-day drama in the eastern Mediterranean. Hours later, Italy announced that an elderly, wheelchair-bound tourist from New York was missing and believed slain in the hijacking, his body thrown overboard. Wednesday night the United States confirmed ''the brutal killing.'' The other 400 passengers and crew members aboard the ship, the Achille Lauro, were reported to be safe. Ship Sails to Port Said The ship sailed into Port Said before dawn today, and port officials said Egyptian security officials wanted to search the liner. The passengers and most crew members were expected to be brought ashore during the search; security officials were reported to be worried that the hijackers might have left a time bomb hidden aboard.

The Government today overhauled its regulation of the natural gas industry in a move likely to increase competition and thus lower prices for residential, business and industrial consumers. In approving the new regulations, however, the Federal Energy Regulatory Commission deferred action on ''block billing,'' the most hotly contested of four major elements of its plan. Block billing would channel larger amounts of inexpensive gas still under price controls to consumers, giving them a price reduction estimated at $5 billion a year. This bonanza would have come at the expense of producers and pipeline companies. In opposing the measure, they argued that it would make the deregulated gas unmarketably expensive and lead to bankruptcies and a major decline in gas exploration.

IT is a mansion, or nearly a mansion, that Arthur and Esther Bramwell did not know they wanted. ''A friend introduced me to the house three years ago,'' said Mr. Bramwell, a deputy county clerk in State Supreme Court in Brooklyn. ''I told him I already had a house nearby that was paid for, that I didn't need it. He said I had to see it.'' The doors in the 12-foot-high parlors are faced in oak on one side and mahogany on the other. Virtually every opening in the late-19th-century house - doorways, windows and fireplaces - is surrounded with highly detailed and intact late-Victorian woodwork. Mr. Bramwell, pointing out the patterned brass hinges on the oak front door and the coat hooks made of sterling silver in the entry hall, is proud of the elegant 12-room, four-story brownstone he now owns in the Bedford-Stuyvesant section of Brooklyn.

Ending a weeklong deadlock, the Senate gave overwhelming bipartisan approval today to a plan to require a balanced budget by 1991. The plan passed 75 to 24, with strong Democratic support, from liberal to conservative. Senate supporters from both parties said the vote would put pressure on the House to respond with its own proposal to avoid political damage just before the beginning of the election year. The Democratic leaders of the House have called the Senate plan unacceptable.

Kidder, Peabody & Company mishandled ''at least $145 million of its customers' fully paid-for securities,'' according to an administrative complaint issued yesterday by the Securities and Exchange Commission. The regulatory agency said the 120-year-old firm used $90 million of the securities to make repurchase agreements and $55 million as collateral for bank loans between late 1983 and late 1984. In effect, repurchase agreements are the pawning of securities for brief periods as a means of raising cash. Rules prohibit a brokerage firm from using paid-for securities for its own use unless the customers agree.

In a sweeping restructuring, Rupert Murdoch plans to consolidate his recently purchased 20th Century-Fox Film Corporation with his six Metromedia Inc. television stations to form Fox Inc., which will include a planned Fox Television Network. The company would say little about the proposed network except that it would not be like the three existing major broadcast networks. Barry Diller, who was made chairman and chief executive of Fox a year ago by its previous owner, Marvin Davis, will be chairman and chief executive of the holding company for Fox Inc., which will be divided into the 20th Century-Fox Film Corporation, the Fox Station Group and the Fox Television Network. Alan Horn, the chairman and chief executive of Embassy Communications, has been named president and chief operating officer of the film corporation, which will include motion picture and television production and distribution, home video production and marketing, and film processing. Mr. Diller remains chairman and chief executive of the 20th Century-Fox Film Corporation.

International The four sea hijackers surrendered in Egypt after a two-day hostage crisis in the eastern Mediterranean. Hours later, Italy announced that an elderly, wheelchair-bound tourist from New York had been slain by the hijackers and thrown into the sea. Egyptian officials said the four hijackers had surrendered unconditionally to a representative of the Palestine Liberation Organization. [Page A1, Columns 3-6.] Word of an American's death aboard the Italian cruise liner was relayed to Prime Minister Bettino Craxi only moments before he was to announce the peaceful conclusion of the hijacking at a news conference. Italians were shocked when the ship's captain, Gerardo de Rosa, described how one of the hijackers appeared on the ship's bridge Tuesday in blood-spattered shoes and trousers. The captain said he believed the blood was that of Leon Klinghoffer, an ailing, 69-year-old American. [A10:1-4.]

Health officials today asked the Government to consider sealing a valley north of here as a mass grave for landslide victims. The authorities, worried that the uncounted bodies cannot be quickly recovered from beneath tons of mud and rubble at the site of the Mameyes shantytown, fear a health threat to the general population. Health Threat Is Seen ''It is one of the most difficult and delicate decisions that the Puerto Rican Government has ever been asked to make,'' said Mayor Jose Dapena of Ponce, a city of 250,000 people on the island's southern coast. ''But it is a decision that has to be made quickly due to the threat to the rest of the community.''

At the home of Leon and Marilyn Klinghoffer, the celebration was just beginning. Giddy with joy and relief, the two Klinghoffer daughters, joined by relatives and close friends, held their champagne glasses aloft, about to toast their parents' deliverance. ''It's confirmed!'' Lisa Klinghoffer, a 34-year-old artist, announced about 11:30 in the morning after the State Department telephoned with the good news. ''It's over!'' ''Call grandma!'' her sister, Ilsa, a 28-year-old health administrator, shouted, bursting into tears. ''I can't believe it! I'm so happy!''

As President Reagan's campaign to overhaul the Federal income tax system draws to a close, some aides and Congressional leaders agree that he has failed to generate public enthusiasm behind the plan. This failure is one reason Congress is highly unlikely to pass a tax-revision bill in 1985. Such passage by year-end has been a chief Presidential objective and the central goal of the campaign. The causes of the failure, according to White House aides and Congressional leaders, boil down to this: a basic overestimation of Mr. Reagan's capacity to mobilize a public groundswell on an issue, tax revision, that is dauntingly complex and on which the public is proving surprisingly cautious.