What was going on when I was born?

Fighting erupted in the mountains east of Beirut Tuesday, reportedly between Israeli-backed Lebanese Christian militiamen and Syrian troops. Plans to begin withdrawing Palestinian guerrillas through that area to Syria today were then postponed. The postponement was announced without explanation by the Israeli military command in Tel Aviv, the Israeli radio reported. The command said the pullout by sea would continue as scheduled.

Cities Service Company stock advanced by $2.25, to $45 a share, in heavy trading on the New York Stock Exchange yesterday as investors showed their anticipation of a higher takeover offer developing for the Tulsa-based oil concern. In a bid that has been rejected by Cities Service, the Occidental Petroleum Corporation has offered $50 a share for 49 percent of the company's stock. The balance would be acquired through an undefined exchange of securities.

A Federal judge today cleared the way for the Government to penalize an American company for refusing to comply with President Reagan's sanctions against supplying equipment for the Siberian natural gas pipeline. The company, Dresser Industries, has declined to tell its French subsidiary to defy a French Government order to deliver equipment to be used for the Soviet pipeline. United States District Judge Thomas O. Flannery refused to grant Dresser a restraining order that would have temporarily shielded it from punishment by the Commerce Department. Cabinet members, considering what penalties to impose, recommended during a meeting held in unusual secrecy that Dresser and Dresser France, the subsidiary, be placed on an American ''denial list,'' according to two Administration sources from different departments. The action would prevent the subsidiary from having any commercial relations with the United States.

A table in Business Day on Monday incorrectly reported a Sun Company offering of debentures. The issue is rated Aa2 by Moody's and AA by Standard & Poor's and is being under- written by Kidder Peabody & Compa- ny, First Boston Inc. and Warburg Paribas Becker Inc.

A Business Day article on Monday about the new tax bill incorrectly described its status. It has not yet been delivered to President Reagan for signing.

SUMMER is running out. In the New York area, you knew it from the nip in the air last Sunday. That means the season for bright-tasting summer vegetables has only about a month to go. And even though we can count on a dependable year-round supply of most vegetables, thanks to efficient long-distance transportation, several varieties of the seasonal bounty are decidedly superior when they come from nearby farms. Ripe, juicy tomatoes; huge fragrant bunches of basil and lively sprigs of other herbs such as tarragon and sage; crisp, unblemished cucumbers, their skins uncoated with shelf-life-extending waxes and oils; tender young zucchini and sweet corn - these are some of the vegetables to stock up on now and to prepare and put away so that the flavors of summer can be enjoyed in less bountiful seasons. One of the best and most versatile of these is ratatouille, a savory @Proven,cal vegetable stew that is best prepared with richtasting, fully ripened tomatoes and fresh herbs. It can be prepared in batches, some to serve within a few days and the rest to store in containers and to tuck into the freezer to become the basis for an almost infinite number of impromptu variations.

FOR Americans with a sweet tooth for candy bars, 1982 has been a very good year. Fierce competition in the candy industry combined with moderation in sugar and cocoa prices has reversed a trend that caused bars to shrink almost to record low weights in 1980. A Hershey's Milk Chocolate Bar, for example, weighed only 1.05 ounces in 1980 and cost 25 cents; today it weighs nearly 1.5 ounces and costs 30 cents, a price decrease of 60 cents a pound. Other major candy manufacturers, such as M & M/ Mars, which announced a 10 percent weight increase in its bars last month, and Peter Paul Cadbury have experienced similar fluctuations and today their products are at their highest weights in years. ''Some of the biggest candy bars ever are being sold today and, considering the price, they are a real value,'' said James Echeandia, a candy industry analyst who is executive vice president of the American Consulting Corporation in Orlando, Fla.

President Reagan took office promising a fundamental change in domestic social policy, and it is now clear that he has achieved much of what he promised. The growth of welfare and food stamp rolls has been halted, even amid a recession with the highest level of unemployment in 40 years. The Medicaid rolls have nearly stabilized. Expenditures for the construction of subsidized housing, for public-service employment and for job training have been sharply curtailed. Congress finished work last week on a second round of domestic spending cuts sought by Mr. Reagan. With that, it has become easier to discern the effect he has had on social policy.

Federal agents have arrested a man in Charleston, S.C., who Bronx authorities believe can provide evidence against two men suspected of being accomplices in a murder four years ago. Howard (Buddy) Jacobson is serving a prison term for the murder. The man, Joseph A. Margarite, 33 years old, had disappeared after the slaying of John Tupper, a former East Side restaurant owner and the fiance of Melanie Cain, a 25-year-old fashion model who had lived with Mr. Jacobson for five years.

In a volatile market session marked by the second-heaviest trading volume in New York Stock Exchange history, the Dow Jones average of 30 industrial stocks ended its recent surge yesterday and plunged 16.27 points. A selling burst in the final hour trimmed prices of some prominent blue-chip stocks as the Dow, in its biggest drop in six months, finished at 874.90. In the seven previous sessions, the market's most closely watched indicator soared almost 115 points as institutions and individual investors, taking heart from plunging interest rates and hopes for an economic recovery, poured cash into equities. In the last five trading days, volume on the New York Stock Exchange has climbed past 100 million shares, a pinnacle never previously reached, no less than three times.

Consumer prices, which had increased by double-digit annual rates in May and June, moderated in July, rising by six-tenths of 1 percent, the Labor Department reported today. Smaller rises in gasoline prices, stable food prices and a decline in the growth of housing costs all helped to slow the rate of inflation. But medical care costs continued to soar. The July increase in the Consumer Price Index brings the annual rate of increase through last month to 5.4 percent, back into the range of 5 percent to 7 percent that many economists are predicting for 1982.

The City Council yesterday voted 35 to 4 in favor of creating a city-run Municipal Utility Service that its supporters said would make low-cost hydroelectric power from the state available to city residents by 1985. The proposal, championed by Mayor Koch, will go before the voters on Nov. 2 in the form of a referendum. The Council voted after a relatively brief debate. Opponents contended that the measure was not well thought out, did not guarantee substantially lower electric costs and might not hold up to a court challenge.