COMPOSERS' anniversaries - which fall like rain this month - sometimes seem to have been invented by publishers and record companies, as Christmas was invented by department stores. And, like Christmas, there comes a point when your heart sinks at the prospect of yet another, poised to hurl its celebration package at you with a merciless assault of sausages on sticks and cheap wine. But there are two kinds of anniversary; besides those which are no more than a claim on market share disguised as a party, there are others which, more interestingly, put an equivocal reputation to the test. Nicholas Maw's 60th birthday, which I wrote about last week, was a good example of the latter; and so is Paul Hindemith's centenary, which fell on Thursday and prompted at least two major observances in London.

Hindemith was the complete musician: a successful conductor, a string- player of international stature (he premiered Walton's Viola Concerto in 1929), an influential music theorist and educator, and a composer who wrote for almost every genre of performance - amateur and professional - as well as every instrument. Double bass, cor anglais, heckelphone and (above all) viola players are eternally in his debt for the contribution he made to their otherwise marginal repertoires. He also emerged from the nightmare of Nazi Germany with clean hands, having offended Dr Goebbels and chosen exile in America rather than compromise. Audiences in the 1950s would have considered him the natural heir to Richard Strauss: the man who, in Ezra Pound's words, "renewed the German mission in music".

Half a century on there are other contenders for that title: namely Karl Amadeus Hartmann and Hans Werner Henze, who are fascinatingly grouped with Hindemith - the Three Hs - in a study just published by Phaidon. In other words, Hindemith's currency has been devalued; and his few concert scores of unquestioned genius - the Mathis der Maler Symphony, the Nobilissima Visione Suite and the Symphonic Metamorphoses after Weber - seem, in retrospect, more than counterbalanced by tracts of austerely pedestrian neoclassical chamber music enlivened by a sense of humour you could only call Teutonic. At the opening of the Hindemith International Viola Festival which has been running at the Wigmore Hall since Wednesday, I asked Lady Panufnik (widow of the composer Sir Andrzei) how her husband would have summed up Hindemith's achievement. "Well," she replied after a diplomatic pause, "he'd probably have said: 'a craftsman'." Alas, nothing we heard that night made any greater claim.

Organised by, and featuring, the Japanese virtuoso Nobuko Imai, the festival was clearly The Violists' Revenge for all those jokes at their expense. Its message was that whatever anyone else thinks about the Cinderella instrument of modern times, Hindemith prized it; and the opening programme accordingly featured his Trauermusik (viola and strings), Der Schwanendreher (viola and chamber orchestra) and Octet (with two violas instead of the usual one! Exciting stuff).

None of this, I'm afraid, said much for the vitality of Hindemith's imagination, even though the London Sinfonietta gave the Octet all (if not more) than it was worth, and the artistry of Imai asserted the gentle but radiant potential of the poor old viola to hold its own in a solo role. Had the Hindemith centenary ended here, we could comfortably have shelved him for the next hundred years.

But ... there is a Jekyll and Hyde duality in this composer; and although it's hard to explain how the man responsible for so much commonplace "craftsmanship" also produced an opera with the majestic power and emotional impact of Mathis der Maler, he did - and it opened at Covent Garden on Thursday, the day of the centenary itself.

This new production is an unmissable event, not least because Mathis has never before had a full professional staging by a British company. In Germany, by contrast, it's the next best thing to standard repertory; and its roots lie deep in the Wagnerian helden tradition. Like Die Meistersinger it stalks the world of medieval artists, glossing history with legend, and hangs on the pageantry of the past a timeless issue of socio-aesthetics. What response, asks the libretto (Hindemith's own), should an artist make to turmoil and injustice in society? Should he cast aside his pen/brush/critical detachment and take up a gun? Or should he keep his distance and his calling as a instrument of record?

Mathis was written in the mid-Thirties as the power of the Nazis was growing, and its political relevance wasn't lost on them. German performances were forbidden, and it finally premiered in Zurich in 1938. But what proved a red light for Dr Goebbels has shone all too temptingly green for Peter Sellars, the polemical director of such deathless opera stagings as the Californian-freeway Magic Flute, who makes his Covent Garden debut with this production. As always with Sellars the period context has been trashed in favour of "contemporary relevance" - ie people with machine guns drinking Coca-Cola - on the assumption that such things somehow enlarge the resonance of the work. They don't. They just reduce it to ephemera: the local imagery of tabloid West Coast life. With the strong but complex conflicts of Mathis (rich v poor, Lutherans v Catholics in early 16th-century Mainz) emasculated into squabbling between anyplace, anytime urban guerillas, Sellars's staging undervalues its material. And with the distorted programme synopsis and the gruesomely hip supertitles that not only hang over the set but run infuriatingly across it, the game is exposed. Sellars doesn't give you what this opera is about. He gives you what he wants it to be about. They are not the same. The final implication of Mathis is that the artist's duty is to himself and his art. You could watch Sellars's production and not realise that.

But there are moments of undeniable beauty about the staging - especially the set (George Tsypin), which is astonishing: a broken, multilayered sculpture that looks like the roof of the Sydney Opera House indecay, magnificently lit by James F Ingalls. And ultimately the music triumphs. Mathis may run on the same sort of neoclassically formal processes as the chamber music in the Wigmore festival, but in this great opera they are as alive as the chamber works are dead. The counterpoint dances through the orchestra, the chorale-like blocked chords swell into magnificence, the melodies soar into transcendence. And it's all superbly served by Esa-Pekka Salonen, conducting, and a strong cast that combines helden force with lyric cultivation. Alan Titus has extraordinary presence in the role of Mathis - to which he's better suited, dare I say so, than Fischer-Dieskau on the EMI recording. Stig Andersen takes the punishing tenor lead of the Cardinal with unfailing finesse. And Christiane Oelze is stunning as the love-interest, Regina. Rarely have I felt so moved, so shaken, so elated by a group of individual performances - or had my feelings for the work of a composer so completely turned around from one day to the next.

Hindemith Festival: Wigmore Hall, W1 (0171 935 2141), ends today. 'Mathis': ROH, WC2 (0171 304 4000), Mon & Wed.