Thankgiving, as O. Henry remarked, "is the one day that is purely American." Its nationality is most evident, perhaps, in the fact that it is a purely artificial holiday, manufactured by proclamation. It was the first and most successful of a procession so long and tremendously varied that someone should proclaim a Holiday Day to honor our habit of cluttering the calender with special events.

The Puritans created Thanksgiving after surviving to reap their first harvest.But it was also timed to compete with what they considered the Popish feast of Christmas, which was outlawed in colonial New England. Perhaps Christmas was due for such treatment. It had been developed, centuries earlier, to coincide and compete with the orgiastic Roman feast of the Saturnalia.

Observances like these are the major league in our national holiday competition, but there is also a large, colorful minor league, a constant striving for status.

Father got that status in 1910, mother in 1914, but mother-in-law had to wait until 1977. What these esteemed forebears have in common, along with children and grandparents, Christopher Columbus and George Washington, is a national holiday of their very own.

But while father and mother (not to mention Washington and Columbus) have an Act of Congress to establish their day, mother-in-law (like the groundhog, or the Halloween witches) is still out in the cold.

No matter. Some holidays are tricks and some are treats; Mother-in-Law's Day, which was observed nationally for the second time just last month, was a little bit of both. Those with the most to gain, mothers-in-law and florists, both think it is showing great promise.

Mother-in-Law's Day was created with a flourish last year, after earlier market-testing in California, Oklahoma and Minnesota, by Florists' Transworld Delivery, the people who send bouquets across continents by telegraph wire. To their amazement (at that time, the day was considered more trick than treat), it helped to sell 300,000 bouquets the first year.

"It was not promoted as a sales event," according to FTD's advertising and publicity director, Allen Smith, "but more for imagebuilding. The idea was to show that flowers are fun and florists have a sense of humor."

They're still counting the returns on the second annual Mother-in-Law's Day, but early reports from scattered precincts look good. Prospects for making it an official holiday are still speculative; a few mayors and governors have proclaimed it in their localities, but Congress is harder to approach.

In the House, where proposals for new holidays are handled by the Post Office Committee, the rule is that at least 51 percent of the House membership must endorse it before it will go to the committee. The Senate makes the first approach easier. Any senator can recommend a holiday and it will go to the Judiciary Committee, but most of them have a way of disappearing in the labyrinthine deliberations of that body.

Of course, an Act of Congress is hardly necessary for a holiday unless you want the banks to close and mail deliveries to stop - as florists and mothers-in-law certainly do not. (Besides, the same effect can be achieved simply by having your day on a Sunday.)

In fact if the wheels did stop turning whenever someone proclaims National Youth Hostel Week (which happens in May) or National Double Talk Week (earlier this month), nobody would do anything but celebrate.

The 2,500 special days, weeks and months which have been proclaimed for 1978, if celebrated consecutively would take more than 38 years for their proper observance.

That informatior comes from Chase's Calendar of Annual Events, published by the Apple Tree Press in Flint, Mich., which has been chronicling holidays and special events since 1957. The 1979 edition, just off the press, shows a significant escalation in the past year: The special events of 1979 would take more than 47 years to celebrate.

Most of these events are sponsored by a civic, fraternal or commercial organization, and the publishers distribute a form which must be filled out by sponsors explaining what is being observed and why. Among the numerous occasions with commercial overtones are more than 40 associated with food, from apples to yams. National Pretzel Week was held recently and a massive pickle observance, running through New Year's Day, is now in progress under the auspices of Pickle Packers International, Inc.

But the profit motive is only part of the holiday mania. Nobody makes a bundle from Sadie Hawkins Day, which originated in the Li'l Abner comic strip and was celebrated Nov. 4, or from Abet and Aid Punsters Day (Nov. 8). National Aardwark Week (March 5-11) contributed minimally to the gross national product, though it may have made some aarvards happy.

Even the traditional holidays have become suspect. When you observe how Congress has shifted the dates around for some holidays, you may begin to wonder whether they mean anything more than a long weekend. Some dates are sacrosanct: New Year's Day has to come on January 1, and nobody so far has dared to tamper with the holiday popularly called The Fourth of July. St. Patrick's Day seems untouchable, and only the groundhog could change Groundhog Day.

But persons of a certain age can remember when something called Armistice Day always happened on November 11 and was very important because it marked the end of war to end all wars. And few Americans, presumably have any vivid memories of celebrating Midsummer Night on June 21. We have long lived in a society that, despite overwhelming evidence, has not accepted the existence of fairies or witches.

Once upon a time (when it was celebrated with an aspostrophe: Hollowe'en), that night was all tricks and no treats. Whatever else it may have accomplished, UNESCO has done an outstanding job of taming the hours that used to be marked by broken windows, stolen trash barrels and overturned headstones. At the same time, our society (particularly candymakers and manufacturers of masks and costumes has found ways to make it profitable - which means it will probably go on forever, whether we believe in witches or not.

Mothers-in-law undoubtedly deserve at least as much as witches - and probably represent a considerably larger part of the consumer market - so the chances are the Mother-in-Law's Day will be with us as long as marriage remains fashionable.

Reactions to this year's observance are beginning to arrive at FTD headquarters. According to Allen Smyth, one Connecticut florist reported that sales for Mother-in-Law's Day were up 100 percent this year over 1977 and congratulated the organization for having "created a new holiday at a time of year when it was really needed."

Mothers-in-law, too, are writing "a lot of letters," Smyth says. "They write to thank us and they say things like 'I got a wonderful letter and a bouquet from my son-in-law. I never knew he cared so much.'"