J. Edward Day, the wisecracking Cabinet officer who gave the nation a million laughs and the five-digit ZIP code as Postmaster General for three ebullient years, died on Tuesday in Hunt Valley, Md. He was 82 and lived in Chevy Chase, Md., 20815, as it has been known since Mr. Day inaugurated the ZIP code system on July 1, 1963.

The cause was a heart attack while Mr. Day was in Hunt Valley for a meeting, his family said.

When President-elect John F. Kennedy picked Mr. Day to be Postmaster General, Washington insiders were understandably puzzled. True, Mr. Day had been active in Democratic politics, first in his native Illinois and later in California, where he became an executive with the Prudential Insurance Company of America, but unlike most of his predecessors as Postmaster General he had not been a party chairman.

Yes, Mr. Day, a University of Chicago graduate who received a law degree from Harvard University and served on a submarine in World War II, was a protege of Adlai Stevenson when Mr. Stevenson was Governor of Illinois, serving first as a legal and legislative assistant and later as state insurance commissioner. But given the Kennedy feelings toward Mr. Stevenson, an undeclared rival for the 1960 Presidential nomination, that was hardly a recommendation.

And yes, Mr. Day was a Kennedy delegate to the 1960 nominating convention, but only after Mr. Day's first choice, Gov. Edmund G. (Pat) Brown, had faded from contention.

If anyone doubted that Mr. Day had been picked because he had a reputation as a highly capable administrator with a quick grasp of complex issues, there was always his own explanation for how he met most of the qualifications for a top spot in the Kennedy Administration: ''I went to Harvard, I served in the Navy and my wife went to Vassar.''

Whatever his qualifications, as the most obscure member of the Kennedy Cabinet -- and a man so unknown to official Washington that by his own account he was either ignored at high-powered cocktail parties or mistaken for somebody's security guard -- the short, sandy-haired Mr. Day began making a name for himself almost as soon as he took his position behind the oversized desk in the huge Postmaster General's office, the Government's largest, ''a lobby looking for a hotel'' as he described it.

Within weeks he was known as the Kennedy Administration's resident wit and house raconteur, a man who claimed to have received hundreds of ''by the way'' letters, which began by offering congratulations and ended by asking for jobs.

Mr. Day said that when a meat association had asked for a stamp honoring the hamburger, he had told the group that ''we had chewed their suggestion over and decided to put it on the back burner.''

As he later recalled it, Mr. Kennedy had given him three goals: reduce the department's $1 billion deficit, improve service and raise employee morale. By most accounts he accomplished all three, slashing the deficit, streamlining mail deliveries and signing the first labor contract with the postal workers union.

By far his most far-reaching act was putting in place the ZIP (for Zoning Improvement Plan) code, an idea postal executives had been studying for years. With the assistance of Mr. ZIP, a cartoon character, Mr. Day sent notifications to every household in the country, the largest mass mailing up to that time.

Despite assurances that the system would speed deliveries and cut costs, the idea was widely denounced. Like the famous New Yorker cartoon in which the dowager tells her travel agent that if God had wanted people to fly jets he'd have had the Wright Brothers invent them, as New York, 21, N.Y., became New York, N.Y., 10021, many Americans seemed distrustful of any system Benjamin Franklin had not personally approved.

Although ZIP codes did streamline mail delivery, the main effect was to revolutionize mass marketing by providing a quick way to pinpoint potential customers by income level.

Mr. Day resigned less than a month after the ZIP code went into effect, not because of the controversy, he said, but because he was finding it hard to get by on his $25,000 annual salary. Mr. Day, who had used his experience as Illinois insurance commissioner to become a high-paid insurance executive, reaped similar rewards when he returned to private law practice with clients that included the Advertising Mail Marketing Association.

In recent years, Mr. Day, who wrote several light-hearted books, including ''My Appointed Round,'' had spent much of his time at his farm in Barnesville, Md., where after an abortive effort to raise yaks his stock included llamas and cows.

He is survived by his wife, Mary Louise; two daughters, Geraldine Zurn, of Erie, Pa., and Mary Louise Himmelfarb, of Oak Park, Ill.; a brother William of Sierra Vista, Ariz., and five grandchildren.

Correction: November 11, 1996, Monday An obituary on Nov. 1 about J. Edward Day, the Postmaster General who introduced the ZIP code system in 1963, misstated the history of his home ZIP code in Chevy Chase, Md. It was originally 20015; later it was changed to 20815.

J edward Day