Mike Goldgar breathed a sigh of relief yesterday.

Goldgar is the executive secretary and co-founder of the National Council for the Observance of Grandparents Day. Yesterday he finally got his wish when President Carter signed House Joint Resolution 244, permanently establishing Grandparents Day as the nation's newest commemorative occasion. Sept. 9 is the big day.

The people over at the National Association of Greeting Card Publishers and the Society of American Florists were quite pleased. The two trade organizations, according to Goldgar, have pumped a lot of money into the Grandparents Day Council. The florists' society is spending $200,000 for radio and television commercials promoting the new day.

The president's action was also good news to Jerry Nagler, a senior vice president of Lewis & Gilman, a Philadelphia public relations firm. For several weeks, Nagier's firm has been flogging the idea that a box of Whitman's chocolates, which it represents, would make an ideal Grandparents Day gift.

So it went in the industries that stand to profit the most from Grandparents Day. Advertising campaigns were planned and marketing materials prepared as the big day, now set for this Sunday, approached. Hallmark printed a line of Grandparents Day cards and this week began running national television commercials promoting them.

But there was one hitch as the card publishers, the florists and others launched their selling campaigns. Their new day did not actually exist. The resolution asking that it be proclaimed had passed Congress, but it languished on Capitol Hill because House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) had rushed home for the August congressional recess before signing it.

Until O'Neill returned and signed it, the resolution could not go to the White House for the necessary presidential signature and proclamation creating Grandparents Day.

As it turned out, Grandparents Day just made it under the wire yesterday when Carter signed the resolution and issued the proclamation. But by that time, the president's action was largely irrelevant. Grandparents Day, according to those who are promoting it, had by then taken on a life of its own in the television and radio commercials and the colorful posters that went up in bookstores and other establishments where an appropriate gift might be found.

The same people also say it is likely to be with us from here on out, growing in recognition, popularity -- and profits for the industries promotion it -- in the years ahead.

The calendar is dotted with similar commemorative days duly noted in presidential proclamations. Some, like Mothers Day and Fathers Day, are firmly embedded in the national consciousness. Others seem to come and go.

If the history of Grandparents Day is any example, whether these occasions ever become widely observed has far less to do with serious congressional intent or presidential action than with aggressive merchandising by industries that stand to profit from them.

In the case of Grandparents Day, there were some natural commercial allies of the event that Goldgar, 59 a retired New York advertising man who now lives in Atlanta, knew how to locate. He wrote to several of them seeking financial support, and while only the greeting card publishers and florists contributed, others recognized the idea for the opportunity it presented.

One was the Whitman Chocolate Co., which turned over Goldgar's letter to Nagler, its public relations man, leading to the flow of material promoting Whitman's candy as a Grandparents Day gift.

"We latched on to it as a result of that," Nagler said, unaware even this week that Grandparents Day did not formally exist.

For Nagler and others involved in promoting the day, the legal technicalities of a signed resolution proclaiming Grandparents Day were less important than whether the idea took hold in the public mind.

The first Grandparents Day was proclaimed last year, but that resolution applied only to 1978. Without waiting for it to become a permanent occasion, as it now is on the first Sunday after Labor Day, the commercial rush was on.

"The reaction of the greeting card people was that whether the president signed it or not wasn't that important," said Norma Halliday, executive director of the greeting card publishers association, which contributed $10,000 to Goldgar's council.

"We thought it was a great idea," said David Weaver of the florists' society, which he said gave Goldgar a "nominal" amount he declined to specify.

Both Halliday and Weaver said their groups played no role in gaining passage of the Grandparents Day resolution, but are delighted it was approved and signed.

Even before the day was officially on the books, those promoting it saw a bright future. Hallmark Cards in Kansas City estimates that, by 1983, Grandparents Day could become the nation's sixth largest greeting card occasion, bypassing St. Patrick's Day and Thanksgiving, among others.

Goldgar sees an equally bright future.

"It's simple arithmetic," he said. "People have four grandparents to two parents.