What was going on when I was born?

The Federal Reserve Board today cut its discount rate to 12 percent from 13 percent, confirming a drop in interest rates as the economy continues to decline. The cut, which is effective Friday, is the second in a month. On Nov. 2, the Fed reduced the discount rate, the interest it charges on loans to banks, to 13 percent from 14 percent. The 12 percent level is the lowest the discount rate has been since last Dec. 5.

In a move to break a deadlock in the Security Council on the election of a Secretary General, Kurt Waldheim said today that he would not compete in any further ballots for the position. But aides to Mr. Waldheim made it clear that he had not abandoned his hopes of being re-elected. They said the Secretary General had decided against undergoing any further ordeal by voting. But he now runs the risk of the divided Council's agreeing on some other figure.

City Comptroller Harrison J. Goldin called yesterday for a sharp slowdown in the hiring of New York City government employees to avoid tax increases that he said could hurt the city's economy. The Comptroller's proposal put him at odds with Mayor Koch, who has said he is contemplating new taxes on personal income for next year. Mr. Koch also is considering a limit on hiring, but he said it would not be as restrictive as the one put forth by Mr. Goldin. ''It appears that his suggestions today go too far in one direction,'' the Mayor said, referring to the Comptroller's reliance on trimming the number of municipal workers and not imposing higher taxes.

International Plans for the Sinai peacekeeping force were pressed by the United States and Israel. They issued a joint statement in Washington governing participation in the projected force that American officials said should clear the way for four Western European nations to take part. But final Israeli acceptance of the formula appeared to be in doubt. (Page A1, Column 1.) A move to break a U.N. stalemate was initiated by Secretary General Kurt Waldheim, who said he would not compete in further Security Council voting on his post. The tactic was designed to introduce new candidates for consideration. If none is successful, aides to the Austrian diplomat said, he will be receptive to an offer to remain when his current term expires. (A1:2.)

The State Legislature voted tonight to override Governor Carey's veto of a bill to repeal a 193-year-old law requiring that all property be assessed at full value. The vote was a major victory for the legislative leaders, who had assembled the compromise measure after several years of sometimes bitter negotiations, only to see their work rejected by Mr. Carey last month. The law will essentially keep current assessment systems intact, but it includes a number of special provisions for New York City and Nassau County. Proponents of the law said it would protect the state's homeowners from tax assessment increases, but opponents said it would leave in place a crazy-quilt pattern of assessment that discriminates against many homeowners. One legislator said that the new law preserves a system that is ''blatantly racist.''

It was something out of the ''Perils of Pauline.'' There was Drew Lewis, the Secretary of Transportation, saying that time had run out for the loss-riddled Consolidated Rail Corporation, that it should be split up and sold piecemeal. Then there was L. Stanley Crane, the railroad's new chairman and chief executive officer, pleading with the Reagan Administration and Congress to wait just a little longer, that recovery was just around the corner. As it turned out, Conrail got the extra time it wanted. It also got the possibility of reduced labor costs and a gradual phasing out of unprofitable trackage.

The United States and Israel issued a joint statement today concerning participation by four Western European nations in the projected Sinai peacekeeping force. American officials said the statement should clear the way for the European nations to take part. The Israeli Cabinet in Jerusalem conditioned its acceptance of the three-page document on European approval Text of joint statement, page A11. of the principles in the statement, which affirms that the Camp David accords are the sole basis for the establishment of the force and the only existing framework for a Middle East peace.

Plastic bags of trash were stacked on the roof of the Waldorf-Astoria Hotel yesterday to keep the building clear of refuse. No progress was reported in the strike by teamsters against private carting companies.

The Amerada Hess Corporation, marketer of Hess-brand gasoline on the East Coast, will join with the Mobil Corporation in making a new takeover bid for the Marathon Oil Company, industry sources said yesterday. Mobil declined to comment; Amerada Hess officials did not respond to phone calls. Reacting to earlier reports that Mobil would soon take a partner to try to overcome antitrust problems in bidding for Marathon against the United States Steel Corporation, Marathon shares climbed $3.50 on the New York Stock Exchange, to $102.50. Trading soared to more than 800,000 shares amid speculation that the new bid might exceed $126 a share, which is Mobil's offering price for a 51 percent interest. U.S. Steel's bid is $125 a share for the same amount. Sources said that court developments were so fluid that no one could say with any certainty how the offer would be presented. There were indications, nevertheless, that an increase in price did not seem a priority option.

Already a major producer of opals and sapphires, Australia is apparently about to become a major force in the world diamond business. In what is known as the Kimberley area, a remote site about 1,500 miles northeast of Perth, Australian companies have opened up mines that could produce up to 25 million carats annually by 1985, three times the amount currently produced by South Africa. But before Australia can profit from this potential, it must overcome two problems. First, the Government opposes any association with De Beers Consolidated Mines, a South African company that markets nearly 80 percent of the world's diamonds through its Central Selling Organization.

The Government has received detailed reports that five terrorists trained in Libya entered the United States last weekend with plans to assassinate President Reagan or other senior Administration officials, according to top Federal law-enforcement officials. The reports, which the officials declined to discuss in detail, have prompted a huge nationwide search for the potential assassins and Americans to whom they might turn for assistance, these officials said. Agents of the Federal Bureau of Investigation and the Secret Service have been deployed around the country in recent days to question Americans who have past links to Libya, including veterans of the Army Special Forces, or Green Berets, who have worked there training terrorists in an operation organized by a former American intelligence agent, Edwin P. Wilson. Search On for Mercenaries Agents of the F.B.I. in the field said in telephone interviews that they had orders to find and question other military veterans who had worked overseas in mercenary operations and were skilled in the handling of weapons and explosives.

''I don't think we'll have to look forward to another shutdown of the Federal Government.'' - Senator Paul Laxalt, Republican of Nevada. (A1:5)