What was going on when I was born?

Take the incumbent Mayor's political autobiography and turn it into a musical revue? People who ought to know about such things told Charles Strouse that it was a crazy idea. Whether they were right or not, ''Mayor'' is already An Event - even before tomorrow's opening. Such premature celebrity leaves the show's creators and producers more than a little anxious. It also delights the man who plays the title role in real life, as he imagines voters humming mayoral tunes all the way to their polling places in September. As the show was going through frantic early-morning and late-night revisions recently, the Mayor and the people who are putting ''Mayor'' on stage retraced the path to the Top of the Gate at 160 Bleecker Street and tried to assess how the musical would be received.

TWO years ago, a little-known American cosmetics company called Cosmair quietly began buying up the supply of customized aerosol cans in Europe, where they were used to package a break-through hair styling foam called mousse. It was a product that had not yet reached America's shores and when Cosmair introduced its Free Hold mousse here six months later, its can strategy paid off: Cosmair got a running start in the market, preventing rivals from posing a challenge for several months until the can supply could be replenished. By then, American consumers were already equating Free Hold with mousse. And although competitors quickly started selling their own products, Cosmair has kept its commanding lead, now controlling about 40 percent of the lucrative $150 million American mousse market. It far outpaces the rivals that patterned their own products on the mousse developed by L'Oreal, one of Cosmair's European owners. Cosmair's aggressiveness in shaping the mousse market, and its move last year past the venerable Estee Lauder into the No. 3 spot in America's $7 billion cosmetics and fragrance industry, has confirmed for analysts and competitors that Cosmair is a force to be reckoned with. Until recently, the products of this privately held company - which include Lancome and L'Oreal cosmetics, as well as Vanderbilt, Chaps and Lauren fragrances - have been better known than its corporate name.

WITH only four pitchers on his squad of 12, Dick Raposa, the head baseball coach at Tunxis Community College in Farmington, needs a pitcher who can do double-duty. Oddly enough, he has one in Kenny Rodriguez, a stylish left-hander who is also a stylish right-hander. Those steeped in baseball lore can go back to the 1940's for a parallel, to Ed Head, a right-handed pitcher for the Brooklyn Dodgers who occasionally threw with his left hand. Mr. Rodriguez, by being able to pitch with either arm, gives the Tunxis team another pitcher. Indeed, Mr. Rodriguez has, at times, pitched most of a doubleheader without putting undue strain on either arm.

Until now Hartz Mountain Industries, the New Jersey-based company that rose to prominence as a manufacturer of birdseed and pet food, has focused most of its real-estate development efforts in the Meadowlands. But with the construction of a 24-story office tower on Madison Avenue at 61st Street, the company is making its developement debut in Manhattan.

The fruits of gentrification in two Brooklyn neighborhoods will be on display next Sunday in house tours of the Park Slope and Fort Greene sections of the borough. In Fort Greene, a neighborhood that in the past has been associated with high crime and rundown housing, tour participants will see carriage houses, cottages and cast-iron buildings, as well as the turn-of-the-century brownstones and row houses for which the area is best known.

The budget plan the Republican Senate passed at 3:08 A.M. Friday is the second revision President Reagan has agreed to in the blueprint he sent Congress in February. Here are the main elements of the package. The Democratic House will start work on its own plan this week.

LAST year's changeover from New York City to state administration of the rent-regulation laws, and the continuing wave of cooperative and condominium conversions, are altering the procedures that landlords and investors must follow to get rent increases. The new procedures do not affect the regular annual increases allowed for rent-controlled apartments and the lease-renewal increases allowed for rent-stabilized units. Rather, they affect the way owners apply for increases when their rental income fails to give a minimum return on their investment, when they make major capital improvements in a building or when they install new refrigerators, stoves or other equipment in a particular apartment. Especially murky are the procedures to be followed in newly converted co-op or condominium buildings in which investors have bought apartments still occupied by rent-regulated tenants. In those buildings, there are, in effect, several landlords - the sponsor, who may own the bulk of the unsold apartments, and the investors who have bought occupied apartments. Special rules apply in such situations.

The wave of classic pitching continued yesterday at Shea Stadium when Sid Fernandez and Roger McDowell combined on a one-hitter that froze the Philadelphia Phillies, 4-0, for the Mets' third straight shutout and fifth straight victory. But the wave of injuries reached a new and dangerous peak when Darryl Strawberry tore ligaments in his right thumb. He will be lost to the team for at least five weeks. Strawberry, the 23-year-old right fielder and the leading home run hitter on the Mets, fell on his right hand while making a spectacular diving catch in the third inning. He was taken to Roosevelt Hospital, where X-rays showed that he had torn ligaments on the thumb. The decision will be made this morning, but it seemed certain that he faced surgery and the Mets faced a month and a half without on of their brightest young stars.

FOR a President re-elected amid record popularity only six months ago, the days threaten to dwindle to precious few in which Ronald Reagan can effect his long promised ''second revolution.'' The President's strategists appeared to have this in mind. Even before returning to face this week the protracted budget struggle, Mr. Reagan allowed them to make a major compromise in an attempt to keep that issue, with its millstone of a deficit problem, clear of the tax revision that they hope to be his crowning domestic achievement. The word that Mr. Reagan, who earlier had taunted Congress with ''make-my-day'' veto threats, was yielding on defense spending came secondhand from Portugal and sent the Republican Senate into frenzied overtime. It approved a drastically revised Presidential budget, by only 50 to 49, with Vice President Bush flown in from Phoenix to cast the tie-breaking vote. The midnight scene was operatic as Senator Pete Wilson of California was trundled into the chamber in a wheelchair, a day and a half after an emergency appendectomy, to vote. Mr. Reagan made the G.O.P.'s night by suddenly giving way after his warnings about stinting on the military and agreeing to hold Pentagon spending to inflationary growth in return for freezing Social Security benefits for a year. Cheers went up as if the Republic had been saved, but it was only one-house budget action.

Forty people were reported killed today when a flash fire swept through a wooden grandstand at a soccer match in the northern English city of Bradford. Forty-three other people, most of them elderly, were hospitalized with serious injuries, and more than 100 others were treated for burns and shock. Millions who were watching the match on television saw the blaze and watched as the police and spectators tried to save a middle-aged man who walked from the grandstand engulfed in flames. Some Officers Weep Later, the police superintendent said, some of his officers cried.

STUART F. PLATT is playing a businessman's game of cat and mouse. He is trying to entice a new supplier into a bidding war for a huge account. The supplier is feigning lack of interest, but this is a buyer's market, and Mr. Platt is feigning harder. ''We're courting,'' Mr. Platt said. ''But we're not begging them.''

ONE moment you're overcome with joy, the next you're moved to tears. How else can one describe the sight of Charles Barkley, the 6-foot-6-inch, 263-pound rookie for the Philadelphia 76ers, as he soars above taller players to engulf a defensive rebound, dribbles down the floor and then bounces the ball off one of his gargantuan thighs trying to make a behind-the-back pass on a three-on-one fast break? Or how about the time he used his uncanny quickness to force a charging call against the guard Craig Hodges of the Milwaukee Bucks, then drive past Sidney Moncrief, a defensive whiz, only to find himself caught in the air with nowhere to go but down because his teammates weren't sure whether he was going to shoot or pass? He did neither and was called for walking, once he landed. Yet in spite of his ability to go from godsend to goat before the 24-second clock expires, Barkley has become the 76ers' most potent force since Moses Malone arrived here three years ago and muttered his now-famous, ''Fo', Fo', Fo','' in predicting how many games each playoff series would last en route to the title. Malone was off by just one game as that team went on to win the 1983 championship with 12 victories in 13 playoff games, matching a league record.