A WEDDING - Academy 6, Avalon 1, Loehmarn's Plaza and White Flint.

Robert Altman fans are disappointed in "A Wedding." The producer-director whose hits included "M*A*S*H," "Nashville," "Images" and "McCabe and Mrs. Miller," has turned to very lightweight comedy for his portrait of a social ritual.

But the contrast between social formula and chaotic reality has been serving comedy well for centuries, and it works here with an odd balance in the elements. The conventional surface remains calm while the turbulence of catastrophic events churns beneath.

The grand-style wedding, with its pretense of family harmony, bridal virginity, love's permanence and expensively formal living, has an odd hold on the American public. People borrow money to give weddings in a stilted style related to almost no one's ordinary lifestyle. Pregnant brides wear white veils. Feuding relatives come together to act jovial. Couples who would not enter the legal complications if there were no divorce promise God to love each other forever.

Why? People always speak of the wedding as being the happiest day in the life of the bridal couple. Being the romantic heroes of a party with a love-story theme, fancier than anything they are used to, is an experience with a tremendous emotional grip.

"A Wedding" films this in all its gaudy splendor. Amy Stryker plays the bride, the daughter of a prosperous trucker, Desi Arnaz jr. is the bridegroom, whose father is apparently connected with the Mafia. So the wedding is everything the greedy dreams of the newly rich can imagine.

And while it goes on, more or less as planned, under the orchestration of a professional bridal consultant, the dreadful lives of everyone involved also go on their course. There is one disaster after another - the maid of honor announces she is pregnant by the bridegroom, the grandmother dies upstairs, the bridegroom is seduced by one of his groomsmen in the shower, and so on, at the pace of one every few minutes - but all are simply the final results following naturally from their previous activities. The comedy of the film consists of having them all happen on one day that was supposed to be "perfect." Not all of these things make sense, a sloppiness that takes away from the effect. It is impossible to conceive, for instance, of the guests a Mafia chief invites to his daughter's wedding deciding at the last minute to snub the event.

There are a number of stars, including Mia Farrow, who seems to be perspiring feverishly in all her recent movies; Dina Merrill as a unflappable aunt; Lillian Gish, as, for most of the film, an eccentric corpse; Geraldine Chaplin as the wedding consultant, a dictator in chiffon. The best of their subplots is a love-at-first-sight encounter between the bride's mother (Carol Burnett) and the bridegroom's uncle (Pat McCormick). And there is rather a triumph to having everyone - nearly everyone - survive the day. That is, after all the attraction of ritual - the erroneous idea that there will be a happily-ever-after.