What was going on when I was born?

The Khmer Rouge rebel movement in Cambodia announced today that its leader, Pol Pot, was retiring from command of the organization. The announcement on the Khmer Rouge radio said that Mr. Pol Pot, who ruled Cambodia for four years until Vietnam invaded the country in 1979, would assume a vaguely defined military advisory role. Diplomats and experts on the Khmer Rouge, whose rule of Cambodia was marked by a reign of terror and policies that left as many as two million people dead, reacted to the report with caution. They said it was unclear whether the announced change in leadership would be substantive.

The Economy President Reagan began his drive for income tax overhaul with a blunt attack on the current system in a campaign-style Labor Day speech. He said that his program would benefit middle-class and working-class families and that it was opposed by special interests. [Page A1.] Members of the tax-writing committees in Congress reported lackluster voter support for tax revision but said they would still work for sweeping changes. [D20.]

Prime Minister Margaret Thatcher announced a sweeping realignment of her Cabinet this evening in a what appeared to be a bold attempt to reverse a recent sharp decline in her political fortunes. With her Conservative Party in third place in the opinion polls and back-bench Tories in Parliament demanding a new initiative, Mrs. Thatcher carried out the most extensive shuffle of her six years in office. She installed new ministers in a third of the governmental departments, and chose a new party chairman - Norman Tebbit, a tough right-winger - to prepare for an election in 1987 or 1988. The major surprise of the shake-up, which had been predicted for months, was the demotion of Home Secretary Leon Brittan. From one of the so-called ''great offices of state,'' he was moved to the Department of Trade and Industry, where he replaces Mr. Tebbit. Mr. Brittan has been criticized as a poor communicator and may have damaged himself by his role in the suppression last month of a BBC television program on Northern Ireland.

When Henry Ford perfected the continuously moving assembly line around the turn of the century, he made mass production and a consumer society possible at the price of forcing factory workers to keep pace with machines. Now -compelled by increasing competition from abroad and concerned about outcries over American quality -major manufacturers are beginning to abandon this cornerstone of the industrial economy. Workers today are being retrained to handle the more thoughtful and complicated operations of the new manufacturing process. They are now permitted to stop what was once the unrelenting moving of parts in order to make sure that quality is maintained, and they are switching tasks among team members to alleviate the boredom of repetition. Robots, instead, are doing the types of tedious and unpleasant chores that Henry Ford's workers once sweated over. All this change is taking place in the pursuit of profit and manufacturing efficiency. But one unmistakable result is an increasing assault on the tyranny of the assembly line.

Junior Beeson was shot on a budding day this summer on a subway ride they used to call the ''Zoo Train.'' The car was crowded, an argument developed and someone pulled a gun. The detective on the case said the incident was ''spontaneous'' and that crime on New York's subways knows no particular zone or time of day. The old Zoo Train, which may be just as safe and just as dangerous as any other ride, is an emblem, as it were, for a system that has been troubled for decades. Starts From Eastchester The train, the IRT No. 5 express, begins its downtown run on the elevated tracks above Dyre Avenue in the Eastchester section of the Bronx, a worn but stable part of the borough near the Westchester County border.

In 1921, Robert Moses, exploring alone in a small motorboat, came upon a stretch of unspoiled beach 25 miles southeast of Manhattan and envisioned a magnificent public park, offering miles of oceanfront and dunes of white sand. In 1985, on the last week of the summer season, two regulars, Pat Michaels, a food broker, and Lou Scotti, a retired private investigator, set up their aluminum chairs on the concrete walk beside the refreshment stand at Parking Field 6 of Mr. Moses' creation, Jones Beach State Park. ''Right now, the flies by the ocean are murderous,'' said Mr. Michaels. ''That's why we sit back here, right Scotti?'' ''Northwest wind,'' said Mr. Scotti. ''Blows flies off the marsh,'' said Mr. Michaels. ''We have our own group of people here at Field 6, the nicest people, they come from all over, right Scotti?'' ''That's it,'' said Mr. Scotti. ''Never a dull day down here, right Scotti?'' said Mr. Michaels, snapping the top off a plastic container. ''You want to try my salad, Scotti?'' Mr. Scotti had eaten. ''We like it better after Labor Day,'' said Mr. Michaels. ''More tranquil. How would you classify the Labor Day crowd, Scotti?'' ''Savage,'' said Mr. Scotti. ''The reason we're regulars,'' said Mr. Michaels, ''very simply, this is the only place you get away from all the nonsense, right Scotti?'' ''That's it,'' said Mr. Scotti. A Lonely Stroll Like Mr. Scotti, anyone who has ever been to some place public, knows that it takes all kinds.

DIAMONDS, whose glitter can kindle the worst and best of human passions and whose unrivaled hardness is indispensable to the tools of industry, have never been taken for granted. But the ever-expanding market for industrial diamonds has prompted renewed efforts to reduce the cost of synthesizing them, and provocative discoveries in several fields of science have resulted. One novel approach to diamond synthesis disclosed recently may be capable of turning out useful stones at very modest cost. Scientists are also taking a fresh look at diamonds for another reason: Those octahedral crystals of carbon so prized by gem collectors the world over seem to contain valuable information about the origins of our earth and solar system. It has begun to look as if diamond is a substance whose character is as cosmic as it is terrestrial.

TYPE A behavior is practically an endemic ''disease'' among the officer-students who attend the United States Army War College. Men and women training for sensitive and demanding leadership positions as commanders and top staff officers, not surprisingly, tend to be aggressive, hard-driving and ambitious, characteristics typical of people said to exhibit coronary-prone Type A behavior. When Type A behavior is extreme and characterized by impatience, quickness to anger and overattention to duties to the neglect of family life, the Army believes it can actually be detrimental to an officer's performance as well as to his or her health. So, as part of an overall effort to improve the health habits of future top-level officers, the War College in Carlisle Barracks, Pa., was an ideal place to test the effectiveness of a Type A behavior modification program among people who had never had heart disease.

A Federal District Court judge in Virginia is scheduled to decide on Tuesday whether to appoint a receiver for the mortgage-banking unit of the EPIC group of companies, which is in default on more than $1 billion in mortgage securities. In an affidavit filed with the court late last week, Robert N. Kemp Jr., president of EPIC Mortgage Inc., warned that the company could lose as much as $500 million if a receiver were placed in charge of its mortgage-servicing operations. He said the threat of such a loss would force EPIC Mortgage into bankruptcy. At a court hearing Friday, the National Bank of Washington, the First National Bank of Maryland and the Federal National Mortgage Association, commonly known as Fannie Mae, petitioned Federal Judge Claude Hilton to grant a temporary restraining order against EPIC Mortgage. Fannie Mae holds more than $100 million in EPIC-originated mortgages. The two banks are trustees for holders of almost $1 billion in mortgage-backed securities sold by EPIC in the last three years. EPIC has missed more than $10 million in mortgage-related payments to the two banks.

After barely one month in office, Peru's new President, Alan Garcia Perez, has shaken this country out of a prolonged mood of pessimism with a burst of reformist zeal unseen here in years. Exuding self-confidence, the 36-year-old President has tackled problemthat had long appeared insoluble, including inflation, corruption, arms spending, narcotics trafficking, leftist terrorism and a seemingly unpayable $14 billion foreign debt. 'Veritable Prowess' Noted Mr. Garcia has sought to restore what he calls the ''authority'' of government and end the vacuum of power through which the country had been drifting under the former President, Fernando Belaunde Terry. ''Who can deny that Alan Garcia has exhibited veritable prowess during his first 30 days in office?'' the independent Lima weekly Caretas noted in an article titled, ''Decision, Perseverance and Daring.'' It echoed a view widely expressed even by those who opposed Mr. Garcia's bid for the presidency.

President Reagan, saying ''I'm back and rarin' to go,'' began his drive for an income tax overhaul today with a blunt attack on the curpent system as ''unfair, unworkable and unproductive.'' In a campaign-style Labor Day speech that marked the opening of a heightened effort by the Ppesident to push for his program, Mr. Reaean stood in front of the brolze statue of Harry S. Truman in the Old Courthouse Square and repeatedly linked the tax proposal to the populism that epitomized Truman's Presidency. Support dor Plan in Congress The plan has been criticized in Congress and elseuhere. Members of the Congressional tax-writing committees say they found lackluqter support for the plan among constituents last month but would nevertheless support it. [Page D20.] Mr. Reagan, standing in the steamy, sun-drenched square, said, ''Our fair share tax program is a good deal for the American people and a big step toward economic power for people who've been denied power for generations.''

''We came on it early this morning. It was just bang, there it was right on top of it.