What was going on when I was born?

As he looks toward the likelihood of another four years of running City Hall, Mayor Koch faces the prospect that his tasks will be shaped by circumstances that appear quite different, and less favorable, than what they seemed only a few months ago. The economy, he acknowledged the other day, is stagnant and the bond market for now seems inhospitable. The future for Federal and state aid is uncertain. Next year may bring a new Governor in New York and, even though Mr. Koch downplays the significance, new people as well in important positions in his own administration.

The Reagan Administration has devised a proposal that would require state agencies to assess most of the personal property and household effects of welfare recipients. Federal officials said today that the proposed rules would reduce cheating by insuring that the assets of welfare families did not exceed the new statutory limit of $1,000 per household. The rules are being reviewed by the Secretary of Health and Human Services, Richard S. Schweiker, and will take effect Oct. 1 if he approves them, as several of his top aides have recommended. Limit Set at $1,000 The Omnibus Budget Reconciliation Act of 1981, signed by President Reagan on Aug. 13, set a statutory limit for the first time on the assets that a family could have if it wanted to receive welfare under the program of Aid to Families with Dependent Children. The limit is $1,000, not counting the family's equity in a home and one automobile. Until the new law, regulations had set the limit at $2,000.

Decoys are indigenous to America; they are also examples of folk art. You may see them in the water and on the beach, in living rooms in Connecticut and in little shops on Third Avenue. If you want to see the best of the genuine article, however, visit the Museum of American Folk Art today. There are 70 decoys, not only for ducks, but for Canadian geese, mergansers, plovers and curlews, from New England, the Illinois River, the Delaware River, Chesapeake Bay and Long Island. Most of them are of wood, carved with simple tools in economical technique, but a few are made of canvas, leather or papier-mache, and they can be seen until Nov. 8. When you tire of decoys, look at the other exhibits in the museum, at 49 West 53d Street, which specializes in all kinds of folk art from Colonial times to the present. It is open today 10:30 A.M. to 5:30 P.M. Admission is $1; for the elderly and students, 50 cents. For information, call 581-2474.

LABOR will have its first parade in New York in 13 years Monday morning, with marching plumbers shouldering their plungers like drill rifles, with pr inters in regulation apron-and -folded-paper-hat uniform, with dozens of floats, 150 bands and members of the several performing unions of the A.F.L.-C.I.O. singin g, dancing and acting their way up Fifth Avenue from 26th to 52d St reet. The Labor Day weekend has over the years been the time for the last weekend in the country, for fairs, art shows and outdoor concerts. And, to be sure, the traditional leisure observances will again be with us -the Washington Square Outdoor Art Exhibit, the Richmond County Fair, entertainments at the South Street Seaport, the Bronx Botanical Garden and Forest Park in Queens. But this year also marks something of a return to the day's original intentions as a celebration of American labor.

Thrift institutions, acceding to the I.R.S., modified the bonus packages they are offering to promote the new tax-exempt savings certificates. They will continue to offer high-interest accounts as an incentive to purchase the certificates, but they will protect the tax-exempt status of the interest on the certificates by making their purchase optional. (Page A1.) The I.R.S. formally withdrew tax exemptions for any certificates bought as a requirement of the bonus plans rather than as voluntary purchases. (D4.) The one-year Treasury bills that will determine the yield on the new savings certificates were sold at a record rate of 15.056 percent on a discounted basis, for a yield of 18.01 percent. This would initally produce a yield on the certificates of 12.61 percent. Treasury bond prices fell sharply, producing yields approaching 15 percent. (D5.)

The Corning Glass Works said yesterday that it would attempt to sell its 23.9 percent interest in the Owens-Corning Fiberglas Corporation. The announcement came after Corning abandoned its previously undisclosed efforts to arrange a friendly merger with the Toledo-based building materials company. Analysts said that the divestiture should bring Corning more than $200 million and might lead to a takeover of Owens-Corning, the company that pioneered the widespread use of glass fibers after it was set up by Corning and Owens-Illinois Inc. as a joint venture in 1938. ''We either wanted to get further in, or get out,'' said Amory Houghton Jr., Corning's chairman. ''They weren't terribly enthusiastic about a merger. It is a great company, and we would have loved to have been a part of it.''

Coast Guardsmen and Federal agents armed with rifles and shotguns seized 33 men and 20 tons of Colombian marijuana early today in raids on a smugglers' beachfront landing ground here and a rusty, 80-foot trawler on Block Island Sound. The raids culminated a three-year investigation by Federal drugenforcement agents and local police officers into a smuggling operation that authorities said was based in a beachfront home here. Arresting officers said the confiscated marijuana had a potential resale value of $12 million. After watching a three-hour unloading operation from shorefront hiding places, Federal agents and the police moved in at 6 A.M. and seized 21 suspects and three tons of baled marijuana at Lafarges Landing on Gardiners Bay, between the forks of Eastern Long Island.

Polish party insists on right to ap- point plant managers A3 Soviet denies maneuvers starting today are unusual A3 World scientists call for freeze on nuclear arms A3 Egypt announces arrests in reli- gious strife A3 Around the World A5 General Around the Nation A6 Fifty Haitian refugees flee deten- tion center near Miami A6 Supreme Court nominee assailed at anti-abortion rally A8 Thousands of letters roll off Am- trak's Night Owl mail car B2 Harlem church to build condo- minium development B3 Many developers expected to sub- mit plans for Times Sq. B3 Glory days are past for Harry Van Arsdale B3 HOLIDAY WEEKEND Monday is Labor Day. Following are holiday schedules: Parade - Begins 11 A.M., north on Fifth Avenue from 26th to 52d Street, then east on 52d Street to Third Avenue. Parking - Sunday regulations in effect; alternate-side rules suspended. Sanitation - No collections; no street cleaning. Federal, state and municipal offices -Closed. Post offices - Closed; only special delivery and express mail delivered. Banks - Closed. Stock and commodity markets -Closed. Transportation - Subways and buses on Sunday schedules; commuter rail lines on holiday schedules; special bus lanes suspended. Libraries - Closed.Schools - Closed.

Stock market prices plunged again yesterday, apparently because of renewed concern over high interest rates. The Dow Jones industrial average, which had managed slight gains on Tuesday and Wednesday, fell 17.22 points yesterday, to 867.01, its lowest level in almost 15 months. ''The market apparently abandoned any hope for near-term relief from high interest rates because of Wall Street's expectations that the Federal Reserve will report a large rise'' in the basic money supply, said Thomas R. Brown, director of research at Butcher & Singer Inc., a brokerage house. The weekly Fed report, due to be released after the stock markets close today, is expected to show an increase of $1 billion to $2 billion in the basic money supply, Mr. Brown said. Such an increase would indicate that the Fed will not relax its current tight-money policy, which has resulted in high interest rates.

For John Durkin, a longtime steel executive, it has meant 12-hour days in a new career, when all he had in prospect was the hope of early retirement. For James A. Sepesky, the Mayor of this city, it has meant adding to services rather than dealing with a fiscal crisis. These two men, as well as 13,000 other residents in this community, which lies 30 miles south of Pittsburgh along the Monongahela River, have been involved in a joint effort of Government and industry the likes of which the steel industry may never see again. The project - the addition of a new rail mill at an old steel plant to make it more competitive - was made possible by Government loan guarantees, most of which came from a Federal agency that the Reagan Administration is moving to phase out. To the Wheeling-Pittsburgh Steel Corporation, the plant's owner, as well as to union leaders and state officials, the project represents a model of a how Government cooperation with private enterprise can revitalize ailing plants.

''I can't think of anything more insane than to carry out the advice of those who say that I should denounce him on a soapbox.

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