What was going on when I was born?

Six different armies were involved in clashes inside Lebanon today, underlining the steady deterioration of the country's security situation. Palestinian guerrilla factions fought with each other and with the Lebanese Army in the central Bekaa, the eastern valley. Israeli troops, battled with Syrians in the southern Bekaa, and other Israelis clashed near Sidon with Phalangist Christian militias, whom they are trying to evict from southern Lebanon. U.S. Envoy Meets With Gemayel The fighting among the various Lebanese groups and occupation forces formed the backdrop for the first foray by the new American special envoy, Robert C. McFarlane, into the Lebanese crisis. The American diplomat, who arrived in Beirut Sunday night, met for 45 minutes today with President Amin Gemayel in the mountain village of Bikfeiya, northeast of Beirut. They apparently discussed new American ideas for obtaining the withdrawal of all foreign forces from Lebanon.

The American Cancer Society announced new guidelines yesterday for breast cancer detection, stating that women 40 to 49 years old should now have regular mammograms. It previously recommended the specialized X-ray technique on a routine basis only for women over 50. The society said the change was prompted by recent improvements in mammography, resulting in much lower radiation doses and better detection ability, and by new demonstrations that mammography was the most effective method of finding early, presumably more curable, breast cancer in younger women. Technical innovations have now made it possible to detect tiny cancers in the dense breast tissue of young women.

The Medical Society of the State of New York yesterday issued its first guidelines for investigating and reporting complaints of incompetence or misconduct by physicians. ''The guidelines mean that any complaint against a physician will not get lost or not be hidden,'' said Donald Moy, the society's general counsel. ''The guidelines are a part of the State Medical Society's commitment to cooperate with the State Department of Health and the Hospital Association of New York State in working out a constructive approach to the issue of incompetent or impaired physicians,'' the society said in a statement.

The Long Island Rail Road and four of its unions announced tentative agreement on new contracts early this morning, averting a strike that had been scheduled to begin at midnight last night. Walter C. Wallace, chairman of the Federal Mediation Board, announced the settlement at 12:30 A.M. today, noting that as a result, ''there will be no interruption of service'' for the line's 145,000 daily riders. He declined to discuss the settlement terms ''at this time,'' noting that the tentative contract agreements remained suject to ratification by the four unions, representing 1,900 carmen, electrical workers, sheet metal workers and machinists, and the Metropolitan Transportation Authority, which oversees the operations of the L.I.R.R. Robin Wilson, the president of the railroad, said he was ''delighted'' with the agreement. He praised the members of the four unions for ''doing a fine job on the railroad,'' and said ''this agreement will assure the continuing improvement in that service.''

Eighteen years ago, when the Federal Government took over the Ocean Manor apartments here through a mortgage foreclosure, the 10-story complex was considered a white elephant - a home for moderate-income elderly people in the shabbiest part of a shabby city. But with the coming of casino gambling, the boardwalk, even in its most decayed stretches, once more became a desirable address. Now the Government, which paid $4 million for Ocean Manor, thinks it is worth as much as $30 million, and like other owners of once neglected properties here, it wants to sell and realize a profit. The prospective sale has frightened the manor's 250 elderly residents, many of whom have already been forced to pick up and move at least once since casino gambling arrived in 1976 - from the President Towers, the Ritz-Carlton and other hotels along the boardwalk.

The Federal Government, in a landmark civil-rights suit that charges discrimination in housing and education, opened its case yesterday against the City of Yonkers and its Board of Education. The Government contends Yonkers has intentionally segregated its school system and subsidized housing along racial lines. Lawyers for the Justice Department argued before Judge Leonard B. Sand in Federal District Court in Manhattan that school officials had ''engaged in an illegal and intentional pattern of racial segregation'' in the system by assigning faculty members and administrators according to the racial compositions of schools. A Justice Department lawyer, Josh Bogin, said the city had significantly contributed to racial imbalance in the schools by building at least 90 percent of its subsidized housing west of the Saw Mill River Parkway, an area that is predominantly black and Spanish- speaking.

The American Stock Exchange and the Chicago Board of Trade yesterday announced an agreement that gives members of each exchange trading rights to the other's stock index products. The agreement would be the first time that members of a stock exchange and a commodities market would be able to trade each other's products without having to register with the respective exchange. Among other products that Chicago Board members eventually may have access to are options on gold bullion that the Amex plans to trade.

International Efforts to restrain the dollar's value in relation to other major currencies were made by the United States and its allies. They intervened for the third successive day in world currency markets with coordinated sales of dollars and purchases of other major currencies. The action marked the first time the Reagan Administration has intervened in the currency markets over an extended period in an attempt to guide foreign-exchange rates. (Page A1, Column 6.) Six armies clashed in Lebanon, underscoring the steady deterioration of the country's security. Palestinian guerrilla factions fought with each other and with the Lebanese Army in the central Bekaa, the eastern valley. Israeli troops battled Syrians in the southern Bekaa, and other Israelis clashed near Sidon with Christian Phalangist militiamen. (A1:4.)

After their worst year in more than two decades, steel manufacturers were suddenly deluged with new orders in February and March. Certain that the worst was over, manufacturers restarted dormant blast furnaces and expressed confidence that 1983 would be the year of steel's comeback. Now it appears that reports of steel's revival were greatly exaggerated. Since April, orders in most sectors of the industry have tapered off, and capacity utilization for raw steel has fallen steadily.

The reports of death sentences in the Soviet Union have a sameness to them. It might be at the end of a tale of embezzlement in a southern republic, or the discovery of another Nazi collaborator in the Ukraine, or a paragraph tucked into the corner of a newspaper under the headline ''From the Moscow Prosecutor's Office and and the Moscow People's Court,'' reporting the conviction of a murderer. ''In connection with the exceptional scope of the crime,'' the article will say, the defendant ''has been sentenced to the supreme measure of punishment, execution by shooting.'' Sometimes it also says, ''The sentence has been carried out.'' Despite occasional efforts to abolish or restrict it, the ''supreme measure of punishment'' has been an almost constant fixture of Soviet justice since the Revolution.

The Bank of New York Company and the Northeast Bancorp of Connecticut announced yesterday that they had agreed to merge, despite a state law prohibiting mergers of New York and Connecticut banks. The merger, the first formal bid by a New York bank to enter the lucrative southern New England market, would enable Bank of New York to move into the prosperous Fairfield County area bordering on New York City's affluent Westchester County suburbs. Some of the nation's biggest corporations are based in Fairfield County, including the Union Carbide Corporation, the General Electric Company and the GTE Corporation. Terms provide that Bank of New York pay 185 percent of Northeast's book value at the time the deal is completed. At the current book value of $37.44 a share, Northeast's stockholders would receive $69.26 a share, or some $200 million for their nearly 2.9 million shares.

Markets The U.S. intervened in world currency markets for the third trading day in a row in an effort to slow the rise of the dollar. It was joined by West Germany, Japan, Switzerland and France. While the coordinated effort may cost Washington as much as $1 billion, some experts said, it could still fail to curb the dollar, which has been pushed upward mainly by rising U.S. interest rates. (Page A1.) The dollar fell slightly in foreign currency trading. Gold prices, meanwhile, rose $2 an ounce, to $414 in New York. (D5.) Long-term interest rates stabilized as the Treasury completed the first part of a three-part, $15.75 billion financing. It sold $6.5 billion of new 11 3/8 percent, three-year notes at an average yield of 11.43 percent. The yield, the highest since August 1982, was further evidence that investors and securities dealers are concerned about the direction of interest rates. (D1.)