What was going on when I was born?

The Nuclear Regulatory Commission said yesterday that, in principle, the Shoreham reactor on Long Island could be allowed to load fuel for low-power testing, even though it may never be permitted to operate commercially. The commission did not give the operator, the Long Island Lighting Company, permission to load the nuclear fuel, but its 3-to-2 vote cleared away a major obstacle. The commission acted after a threejudge licensing board, which has been hearing the Shoreham case, said that uranium should not be loaded before there was ''reasonable assurance'' that a dispute over emergency planning would be resolved and full operation allowed.

The third and final stage of President Reagan's income tax cut, the keystone of his economic program, takes effect today, and is expected to save taxpayers $30 billion over the next 12 months. Many economists believe the tax cut will stimulate consumer spending and an economic recovery in the second half of 1983. (Page A1.) Mr. Reagan has delivered both personal income tax reductions and an improving economy, but they follow a severe recession. (B9.) Bank regulators decided to eliminate all remaining interest rate ceilings on savings certificates purchased after Oct. 1, and to ask Congress to permit banks and thrift institutions to pay interest on checking accounts held by businesses. The decision eliminates interest rate limits on the popular six-month savings certificate. (D1.)

Norton Simon Inc., the big consumer products company, has accepted a sweetened takeover bid from Esmark Inc. estimated at $918 million, the companies announced last night. The announcement, telling of a definitive agreement, followed marathon negotiations Wednesday and yesterday between David Mahoney, chairman of Norton Simon, and Donald P. Kelly, chairman of Esmark, a food processor and manufacturer. Esmark's new two-step bid was valued at an average price of $34 a share in cash and stock, up $1.75 a share from its earlier proposal. Wall Street analysts said they expected the deal to go through, although there was always a possibility that other suitors might yet outbid Esmark.

The first half of 1983 was a time of plenty on Wall Street, with the vast majority of stocks surging upward in a continuation of the powerful bull market that began last summer. But even in a bull market, there are losers. While most investors were making their fortunes, an unlucky few lost a lot of money. The biggest winner on the New York Stock Exchange so far this year has been the International Rectifier Corporation of Los Angeles, a little-known maker of semiconductors and pharmaceuticals that recently invented an inexpensive, highly efficient technique to power silicon chips and other electronic devices.

Mayor Koch announced yesterday that he was abandoning his policy of trying to collect property taxes from many cultural and other nonprofit organizations. The Mayor said he had decided that the path of ''good sense'' required him to make it easier, not harder, for as many as 300 such groups to gain tax exemptions. He said he would propose legislation to the City Council that would allow those groups to qualify for exemptions as ''educational'' or ''charitable'' institutions. The organizations that would benefit range from large museums and foundations to tiny storefront organizations that serve ethnic and racial groups.

Because of a mechanical error in some copies yesterday, type was misplaced in the final paragraph of a news analysis article on the ethics of acquiring Carter campaign materials. It should have read: ''People on the inside of both administrations agree that public attention will not abate in the current situation until key questions are answered. They also agree that the ethical and moral questions will not be answered until it is known for sure who let the documents out, and why.''

The third and final stage of President Reagan's income tax cut, the keystone of his economic program, becomes effective Friday. It is expected to save taxpayers $30 billion over the next 12 months. Paychecks will start to look different in the next week or two as employers adjust withholding to reflect the final 10 percent cut in income tax rates. The extra take-home pay is expected to stimulate consumer spending and the nation's economic recovery in the second half of this year.

Kevin Kline is the cunning protagonist in ''Richard III,'' the only Shakespeare production this season by the New York Shakespeare Festival, which has begun previews at the Delacorte Theater in Central Park. There's no official opening date yet for the play, which will run through July 31 at 7:30 every night except Mondays and July 21. Free tickets are distributed starting at 6 at the theater, but people usually start lining up hours before, many horizontally on picnic blankets. The Delacorte is reached best from the entrances at Central Park West and 81st Street or Fifth Avenue and 79th Street. VIDEO WORKS AT THE MODERN ''Bad Smells,'' a video work by the choreographer Twyla Tharp, is, she says, ''about nature's darker forces, which would be called evil if there were a way to judge.'' This work, which has superimposed dance images, is one of six video performances that can be seen - one a day - today through Aug. 16 in the Video Gallery at the Museum of Modern Art, 18 West 54th Street. The pieces, 15 to 30 minutes long, are repeated all day. Fridays, the work is John Sturgeon's ''Spine/Time.'' Saturdays, ''Secret of the Waterfall'' by Charles Atlas and Douglass Dunn, shot on Martha's Vineyard and described as elegant and humorous. ''Bad Smells'' on Sundays. Free with museum admission: $3 ($2 for students, $1 for children and over-65's). Information: 708-9500.

The Synthetic Fuels Corporation announced today that it would provide $120 million in guarantees for a California project to convert coal to gas, the first financial commitment the corporation has made since Congress created it in 1980. The corporation, which has already spent nearly $50 million on administrative costs, said after a board meeting that it would support the Cool Water Coal Gasification Project already under construction in Daggett, Calif. The plant will burn synthetic gas to generate electricity equal to the power produced from 4,300 barrels of oil a day. Initial electric output at the $300 million plant is expected next year.

A chart in Business Day yesterday with an article on home health care was labeled incorrectly.

Bank regulators decided today to eliminate all remaining interest rate ceilings on savings certificates purchased after Oct.1, 1983, and to ask Congress for legislation to permit banks and thrift institutions to pay interest on checking accounts held by businesses. Some checking accounts for individuals already pay interest. The action means that from Oct.1, there will be no Federal limit on the interest rates that financial institutions can offer on certificates with maturities from 32 days to 2 1/2 years, including the popular six-month savings certificate. Other maturities have already been deregulated. The regulators left unchanged the ceilings on the interest paid on passbook savings.

A construction company executive testified yesterday that his company had complied with state requirements on hiring minority subcontractors by forming a temporary partnership with a black contractor who had no office, no employees, no equipment and no funds. The testimony, on a $30 million contract at the New York City Convention Center, came as the State Commission of Investigation opened hearings into reports of abuses under the state's Minority Business Enterprise Program. The commission was also told during the hearing, at the State Office Building, 270 Broadway, that the Metropolitan Transportation Authority had fulfilled requirements to employ Hispanic companies by awarding contracts to a prosperous architectural firm headed by a native of Colombia. Citing the convention center arrangement and other cases, Lola S. Lea, the chairman of the commission, said, ''By forming joint ventures with sham minority-run businesses or by setting up 'front men,' successful white firms are depriving minority contractors of job opportunities.''