What was going on when I was born?

Torrential weekend rains and overflowing rivers swamped wide areas of Connecticut yesterday with the state's worst floods in decades. The state police said that eight persons were dead or missing in the storm. More than 1,300 others were removed from their homes as floodwaters invaded residential areas, washed out roads and earthen dams and disrupted electric and telephone service and public transportation for tens of thousands of residents. The floods, accompanied by 5 to 8 inches of pounding rain, struck a wide swath of the state, from Westport and other Fairfield County communities on the west to Waterford and New London on the east. At least 38,000 homes were hit by power blackouts, and 6,000 telephones were knocked out. Nearly all trains in the state, including those operated by Amtrak between New York and Boston, were halted as Conrail and Amtrak used buses to carry passengers. Commuters and long-distance travelers were expected to face further delays today. Many communities in flooded areas canceled school for today.

The Security Council unanimously demanded tonight that Israel pull its invading forces out of Lebanon. There was, however, no indication that Israel would pay any more attention to this order than to the unanimous Council demand Saturday night for a cease-fire. Instead, Yehuda Z. Blum, the Israeli delegate, taunted the Council's 15 members for ''evincing not the slightest interest'' in scores of terrorist acts he attributed to the Palestine Liberation Organization. ''How many Israelis have to be killed by terrorists for this Council to be persuaded that the limits of our endurance have been reached?'' he asked rhetorically. ''Israel cannot expect this body even to deplore P.L.O. barbarism against Israel's civilian population, let alone take any steps with a view towards curbing that barbarism.''

The rate of decline in the economy increased sharply in May, casting further doubts on a recovery until later in the year at the earliest, according to a monthly survey of purchasing managers in the nation's leading industrial companies. ''This could be the dark before the dawn, but we really have no evidence of that at all,'' said Charles T. Haffey, chairman of the National Association of Purchasing Management, whose index of economic conditions is closely followed by the Government. ''At the purchasing managers' conference they were all saying the second half would be better, but that is on the opinion side,'' he added. Production, incoming new orders and employment fell off sharply from April, amounting to ''the worst performance for new orders and production since December,'' the association's study said. The rate of decline in the economy has been improving since December, ''but May definitely reversed the trend,'' the study said.

International Israel invaded southern Lebanon by land, sea and air in an attack aimed at destroying the main bases of the Palestine Liberation Organization. More than 250 tanks and armored personnel carriers backed up by thousands of infantrymen rolled past observation posts of the United Nations peacekeeping troops and spread out over the frontier, according to a United Nations spokesman in Beirut. (Page A1, Column 6.) The Israeli Army was ordered to push Palestinian forces northward to a distance of 25 miles from Israel's border so that the P.L.O. artillery would be removed beyond the range of Israeli territory, Prime Minister Menachem Begin said in a letter to President Reagan. (A1:5-6.)

''Our computers are down, our telephones are out and there are four to five inches of water on the floor.'' -Warren Silverzahn of the National Weather Service in Bloomfield, Conn. (B4:2.)

With cracks of thunder accenting the most dramatic moments, Mats Wilander completed a memorable performance in the French Open tennis championships today by emerging as the youngest player ever to win the men's title. The 17-year-old Swede defeated Guillermo Vilas, 1-6, 7-6, 6-0, 6-4, in a match that went 4 hours 47 minutes on the slow clay. The time was five minutes longer than the previous record for a final set in 1929, when Rene Lacoste beat Jean Borotra. Wilander, ranked 18th in his first full year as a professional, received $66,000. He beat four players in the world's top 10 - Vilas, Ivan Lendl, Vitas Gerulaitis and Jose-Luis Clerc.

A move is under way in Congress to curtail sharply or even eliminate the Commodity Futures Trading Commission's authority to approve stock index futures, an action that commodities industry officials fear could be the first step in removing the agency's responsibility for regulating financial futures. At hearings opening on Monday, Representative John D. Dingell, Democrat of Michigan, the chairman of the House Energy and Commerce Committee, is expected to open an attack on the commission's recent approval of proposals by several major commodities exchanges to trade stock market index futures. Stock index futures contracts allow a purchaser to speculate on the level of the index over a fixed period of Commodity exchanges are struggling for dominance in trading of stock index futures. Page D3. time. The value of such a contract rises or falls as the index fluctuates, and speculators or hedgers can profit by correctly guessing its future direction.

Prime Minister Menachem Begin said Sunday that the Israeli Army had been ordered to push the Palestinian forces northward to a distance of 25 miles from the Israeli border, to place their artillery beyond the range of Israeli territory. Mr. Begin made his statement in a letter to President Reagan, excerpts of which were reported on the Israeli radio. The Cabinet, after an emergency session, issued a statement saying Israel would not attack any Syrian forces in Lebanon or Syria unless the Syrians engaged the Israelis. The Damascus radio said the Syrian Army was battling the Israelis near Hasbeya, 10 miles north of the border. The Israeli military spokesman said there had been no verification that any such clashes with the Syrians had occurred.

The eighth summit conference of the industrialized nations reached limited agreement today on two contentious subjects - East-West trade and the handling of currency fluctuations - and produced something of a breakthrough on North-South relations. The agreements themselves were the subject of some disagreement: whether they bridged or merely papered over fundamental differences. Prime Minister Margaret Thatcher of Britain described the atmosphere as one of unanimity. Prime Minister Pierre Elliott Trudeau of Canada called it ''difficult.''

Israel's invasion of Lebanon came today as the culmination of months of military and political calculation in which Prime Minister Menachem Begin repeatedly allowed the troops to be massed and the saber to be rattled, only to pull back at what seemed like the last moment. Until today the crucial factors favoring a major assault never quite lined up, and the risks seemed greater than the potential benefits. This time, however, Mr. Begin decided to make the military gamble and to pay the political costs that he and his advisers know exist. The crucial reason was the intensive shelling of northern Israel by forces of the Palestine Liberation Organization, which began Friday afternoon after Israeli air strikes on Palestinian bases near Beirut. The Israeli command described the air raids as retaliation for the shooting Thursday of Israel's Ambassador to Britain, Shlomo Argov, who was critically wounded in London. Five suspects, all traveling on passports from Arab countries, were captured. The P.L.O. denied any responsibility for the attack.

When William Miles bought his new Cadillac El Dorado in August 1980, he chose a diesel-powered engine. Gasoline prices were on the rise and 1978's shortage scare was fresh in his mind. When he sells the car in the next few months, Mr. Miles is not sure what model he will replace it with, but he is sure that the new car will not be a diesel. ''It isn't because I'm unhappy with it,'' he said. ''It just seems like such a contradiction: You pay so much for a car like this and save a few pennies at the gas pump.''

The summit conference ended with limited agreements on East-West trade and the handling of currency fluctuations and also opened wider the subject of North-South relations. But many analysts hold that even the limited gains were produced by papering over fundamental differences. (Page A1.) What gains there were turned out to be more modest than Washington had hoped for. (D6.) There was considerable feeling that the seven leaders did not really come to grips with the worldwide recession. (D6.)