Verbier Festival, Verbier, Switzerland

When that sacred monster Martha Argerich gives a recital, it's news. When she's joined in concert by Evgeny Kissin - a pianist of comparable gifts and elusiveness - it seems a reversal of nature, because these are essentially solitary animals. Now add that inscrutable thoroughbred Mikhail Pletnev to make six hands, and the new Chinese whirlwind Lang Lang to make eight.

Then - if your imagination hasn't by now been blown out of the Milky Way - increase your manual strength with the assistance of Emanuel Ax, Leif Ove Andsnes and James Levine, and here you have the extraordinary gathering in a tent on top of a mountain on Tuesday night. It's never been seen before, and probably never will be again: the living gods of the piano in one massive posse, each riding a shiny black steed. Little wonder the media had flown in from all over the globe.

Yet, when the lights went down, we were transported to an unexpected heaven, with Argerich and Kissin delivering Mozart's late sonata, K521, with immaculate grace. After the first movement, which went like the wind, they established a sweetly confidential tone, bringing out all the orchestral richness of its colouring. An odd couple, you might think, but this was an ideal musical symbiosis.

But what was this "Birthday Festival Orchestra", which then launched into "Happy Birthday To You"? Step forward a no- less-galactic crowd of string players led by Gidon Kremer, and just about everyone who is anyone in the Russian-Jewish diaspora. To see Vadim Repin, Mischa Maisky, Yuri Bashmet - plus that oriental bloom Sarah Chang - all working humbly in harmony was to witness nature being reversed again.

In Peter Heidrich's arrange-ment, "Happy Birthday" went through some wonderfully clever transmogrifications, via the way that Haydn and Beethoven might have treated it, to versions for tango and csardas. To remind us what masters they all are, they then surrounded four of the piano gods to deliver an Olympian version of Bach's four-keyboard concerto. Here we were again in heaven.

The birthday in question was that of the Verbier Festival, dreamed up 10 years ago as an answer to stuffed-shirt Salzburg, and powered by a combination of serious hedonism and artistic quest. Since its begetter, Martin Engstroem, also calls the shots for Deutsche Grammophon, that company's stardust has brushed off all round. The previous day, we'd watched the soprano Barbara Hendricks battling heroically with a programme of Nordic songs under the deafening thunder of rain on the tent's roof. Now, people were letting their hair down.

So, on rolled eight Steinways and the fun began, with arrangements of Rossini, Wagner, Rimsky-Korsakov, and American marches galore. Devoting an evening to arrangements was an interesting idea: as that master-arranger Busoni observed, since every interpretation is in effect an arrangement, we should not look down our noses at them. Moreover, the aforementioned Bach was, for all its sublimity, but an arrangement of a concerto by Vivaldi.

But here we were, in surround-sound. At times, the eight virtuosi dovetailed so deftly that one might have thought that there were only two instruments; at times, they capitalised on the situation, conversing antiphonally as though addressing each other from separate mountain-tops. "The Ride of the Valkyries'' became a wall of sound, the Bumble Bee's Flight was crazier than usual, but eight instruments definitely diluted the "Maple Leaf Rag". And while Pletnev looked stern, Lang Lang was in his element, and the white-tuxedoed Kissin emerged as decorous master of ceremonies, one became aware of the fact that Argerich was nowhere to be seen. Had she ducked out at the last moment because she doesn't do circus tricks? If so, I wouldn't blame her. Many hands make heavy work, and the magic of the evening was concentrated in the first two pairs we heard.

Will this stunt be repeated? I doubt it, and I doubt that it should. But the Mozart was so treasurable that I hope it's put on record. There was no rain, no coughing, and no planes overhead, apart from one in the final movement, flying very high.