LEAD: Anniversaries are a funny business. Sacred to some, they mean nothing whatever to others. Four weeks ago, I went around telling people that Aug. 4, 1989, was the 75th anniversary of the outbreak of World War I. They yawned. Having learned my lesson, I should have hesitated to tell them that today, Sept.

Anniversaries are a funny business. Sacred to some, they mean nothing whatever to others. Four weeks ago, I went around telling people that Aug. 4, 1989, was the 75th anniversary of the outbreak of World War I. They yawned. Having learned my lesson, I should have hesitated to tell them that today, Sept. 3, 1989, is the 50th anniversary of the outbreak of World War II, were it not that this anniversary, unlike the other one, has aroused widespread comment.

As it happens, Sept. 3, 1939, has an auxiliary meaning for me. It was on that day that the Musee d'Art et d'Histoire in the old section of Geneva got ready to dismantle what may have been, each for each and canvas for canvas, the single greatest loan exhibition of European painting that there has ever been.

A big claim, certainly. But what else can we say about an exhibition that included - just for starters - Velazquez's ''Meninas,'' ''The Dream of Philip II'' by El Greco, the clothed and the unclothed Majas and ''The Family of Charles IV'' by Goya, ''Venus, Love and Music'' and the equestrian portrait of Emperor Charles V by Titian, ''The Garden of Love'' by Rubens, a group of Venetian portraits by Tintoretto and Mantegna's ''Death of the Virgin''?

Something of the white magic of that exhibition lay in the fact that a European war was known by then to be both imminent and inescapable. This might well be (and for many visitors it was) the last great exhibition that we would ever see. It was partly for that reason that people of every age, condition and stripe made their way to Geneva, where the run of the exhibition was extended until Sept. 8, 1939. All over Europe, throughout July and August, people said to one another ''You've got to come. Don't demur. Don't delay. It's the last great adventure. Get off your backside and come!''

This critic - at the time a mere nursling - was carried away by the call. He has never forgotten the long journey from Paris by road. The stupendous meals in half-empty restaurants, the demoralized hoteliers and the daily hunt for a newspaper that would bring reliable news. What we said to one another about painting, during that journey, may well have been goofy, by the standards of 50 years later, but I can reel it off to this day, word for ecstatic word.

And I still have, someplace, the starveling catalogue with its plain white boards, its twinned heraldic emblems for Madrid and Geneva, its few and scratchy black-and-white illustrations and its simple but irresistible title: ''The Masterpieces of the Prado Museum.''

Titles of that sort do not always speak true. But this one did. Let the figures speak for themselves. Among the 174 paintings on view were 38 by Goya, 34 by Velazquez and 26 by El Greco, 4 by Murillo, 5 by Jose Ribera, 2 by Zurbaran, 3 by Hieronymus Bosch, 6 by Raphael, 9 by Rubens, 7 by Tintoretto, 10 by Titian, 2 by Veronese and 3 by Rogier van der Weyden.

You will get the general picture. You may not, however, be aware of the superfine quality of everything that was on view. This was not a grudging anthology of the kind that now passes muster in diplomatic terms. It was everything that we had ever hoped to see from the Prado -not least, a masterwork or two that seemed, like the self-portrait by Albrecht Durer, the English portraits by Van Dyck, the Bruegel ''Triumph of Death,'' the ''Adoration of the Kings'' by Hieronymus Bosch and the triptych by Hans Memling that had belonged to Charles V, to be offered to us as a kind of valedictory bonus.

In the ordinary way of international exchanges, no such exhibition could have come about. Nor should it. For a European nation to send abroad so large a part of its patrimony would be indefensible in normal times. But those were not normal times. Civil war had broken out in Spain in 1936. Emergency measures had to be taken. (Among them, at a time of high emotion, was the appointment by the lawful Government of Spain of Pablo Picasso as absentee director of the Prado.) Before long, it was clear that this was a war that would be fought to a finish. The masterpieces of the Prado were sent first to Barcelona, and later to Figueras, a town in northernmost Catalonia. From Figueras it is possible to go to Geneva by train, though the journey was anything but expeditious.