If New York's art galleries are shut, it must be August

Almost the last thing on the minds of most New York art dealers during the month of August is art.

If they are thinking of it at all, it is probably while strolling through a museum in London, visiting a gallery in Berlin, or sunning themselves on a beach in southern France. It doesn't so much matter where they are when they do so, so long as it is as far away from New York's Madison Avenue or SoHo as they can get.

The New York art world, in short, has pretty much shut up shop and gone on vacation.

Not everyone, of course, has left the city. Enough galleries have remained open to satisfy most art-loving visitors to the Big Apple, and enough fair-to-good art is available for looking and enjoying - and for buying, should one be so inclined.

In addition, some of the galleries that are closed have posted discreet signs indicating that their art can be viewed ''by appointment.'' And a few galleries that are technically closed have kept a junior staff member on the premises in order to answer questions, and, they hope, to make a few minor sales.

Most of the ''big guns'' of the art world, however, have scattered the winds. For some, the early summer months are ideal for travel to other world art-centers to see what the international competition is up to, to attend sales, and to check out new talent.

August, on the other hand, is ideal for socializing - both on a personal and on a professional level. Special business matters, such as setting up exchange exhibitions, helping to further a young American artist's career by arranging for his inclusion in a major Italian exhibition, or getting permission to show a ''hot'' new European talent in New York, can be all more easily and pleasantly arranged if the surroundings are comfortable and the atmosphere informal.

At home, meanwhile, these dealers' galleries remain closed with the art stock in storage bins or safely lodged in nearby warehouses. If there is any activity at all, it probably has to do with cleaning and redecorating the premises. Since August is the only prolonged period during which the gallery is closed, the gallery staff may well be taking advantage of that fact by painting the walls or putting in new floors.

But if the commercial art world has shut down to a large degree, the museum world is as busy as ever with foreign and American tourists taking advantage of their vacations to visit whichever of the local museums strikes their fancy. (Or , as some do, showing up at each museum in turn at opening time, and spending their entire vacation looking at art.)

Of all the museums, the Metropolitan seems to be bearing the main brunt of this invasion, possibly because its holdings are so huge, famous, and varied. On the other hand, the Museum of Modern Art, which is normally as busy as any museum in the world, has had to curtail its exhibition schedule somewhat because of its building and expansion program.

The Cloisters, serenely situated atop a high hill overlooking the Hudson River, is another particular favorite of museum visitors who come not only to enjoy its rare and beautiful Medieval art treasures, but to stroll, enjoy the view, and picnic on its grounds as well.

Both the Guggenheim and the Whitney Museums have special summer shows, and are doing business as usual. That is also true of such smaller but nevertheless excellent museums as the Frick Collection, the Jewish Museum, the Studio Museum in Harlem, and the National Academy of Design - to name only a few of the most prominent. And the chances are excellent that all other, more specialized museums, are open as well - although some may be observing summer hours.

Even so, August is not the ideal month to come to New York for art, especially if one wants to get a clear and full picture of what new things are happening in painting or sculpture, or to find a wide selection of art from which to buy. Those relatively few galleries that are open, are probably showing one or two examples of each of their artists, or are using this ''dead'' period to test new talent.

All in all, the move in these galleries reflects the mood of the city during August: sleepy, strained, and somewhat testy.