SAYING goodbye to the century looks set to become the preoccupation of concert-programming for the next three years; and every time a new valediction to modernity surfaces, it prompts the question "Where does modern music start?" It's not the hardest question in the world ("Where will it end?" would be more difficult), but there are several possible answers - including the day in June 1912 when two men sat down at a piano, and gave the first, duet performance of a piece that would premiere orchestrally the following year. The piece was The Rite of Spring; the men, Debussy and Stravinsky. Their seminal encounter was recalled in London this week - at the start of two separate festivals for the composers - with a stunning performance of The Rite, programmed up front as a sort of call to arms.

To be exact, these are festivals within a festival. Sounding the Century is scheduled - naturally - to run through the next three years, and will involve all the main UK orchestras. Organised by the BBC, it's partly an umbrella for things that would have happened anyway, like Simon Rattle's CBSO "Millennium Series", and the Royal Opera's Palestrina. But there are also bespoke events centred on the BBC's in-house resources; and the whole thing launched last Sunday with a Stravinsky night, from the BBC SO under Pierre Boulez, that was so obviously destined to be an "occasion" that it was televised live.

Why Stravinsky? Because he spoke for 20th-century music as Picasso spoke for 20th-century painting: a universal figure (Russian, French, American ... ) whose active career ran from the early 1900s through to 1968, and was always with the times - as reflective and summa- tory as it was innovative and aspiring.

Why Boulez? Because no one else in the past 50 years has exercised so powerful an influence on contemporary music-making. With his gaze fixed on the heaven of tomorrow, Boulez has become the patron saint of modernity, interceding at the highest level with its gods. And that of course includes Stravinsky, whose work he has conducted, selectively but with distinction, for a long while. Especially The Rite.

It was an inevitable choice for Sunday's concert. Programming it alongside two softer, more conventional Stravinsky scores of the same period - The Nightingale and King of the Stars - drove home how, with this piece, the composer sprang like a lion out of nowhere (aka the skirts of Rimsky-Korsakov) dripping blood. It also went some way towards solving the problem of how you re-inject a sense of shock into now-familiar music. Stravinsky once joked that the dauntingly high-register bassoon solo which opens the piece should be transposed up every year to stop players getting complacent about it. He wanted the effort to register. But 85 years on, The Rite just isn't such an effort any more, for players or for listeners. So what does a conductor do?

Boulez's solution is, as always, radical. He plans his kill meticulously, aiming for the jugular, but with a scalpel not an axe. And if the result is a performance that holds back from extremes of volume and pace, its impact comes in the sheer eloquence of its barbarity - building excitement through exactness, with the chilly glamour of a glint of sunlight on a dagger. People say Boulez has softened since he reached his seventies, and it's partly true: there was an uncommon lyricism in this Rite - beautifully delivered by the BBC SO - which made more obvious than usual the folk- derivation of the melodies. But melody isn't so important here as pulse and rhythm, a reordering of priorities that back in 1913 was The Rite's chief gift to the new age. And it was the brilliant but rock-solid rhythmic strictures of last Sunday's reading that proved Boulez is still the arch grand- master of control. A man who, with the curt flick of a finger or the merest turning of a hand (he never has a baton) brings a hundred players instantly to heel.

Contrast Michael Tilson Thomas, who conducted the LSO through Thursday's opening concert in the Debussy festival, Painter of Dreams, at the Barbican. His idea of Debussy is all freedom, un-policed; and it has to be said that the dividing line between freedom and uncertainty can be thin. The Nocturnes that began the programme struck me as hesitant, as though the LSO were feeling their way out of the weeks of Brahms they've been immersed in recently. But no matter. Tilson Thomas is peculiarly sensitive to the ethereal, elusive sound- world that makes Debussy another candidate for Founding Father of Modern Music, quite apart from his pianistic contribution to that Rite of Spring duet; and there were interpretative insights here of genuine truth and beauty.

The intention in the series is to programme less- and well-known pieces side by side. Thursday was a mite untidy with short items - Rhapsodies for clarinet and saxophone, the Berceuse Heroique - that were no more than satellite scores, chewing the cud of ideas more effectively worked through elsewhere on a larger scale. But the Trois Ballades de Francois Villon (Debussy in "antique"mode) was a joy, magnificently sung by the rising young Swedish baritone Peter Mattei. And the overall highlight was La Mer, one of Tilson Thomas's calling cards. He did it at the Barbican only last autumn, when he brought his new orchestra, the San Francisco Symphony, here on tour; my reservation then was that it made too big a splash. A Tilson Thomas/LSO performance would, I thought, be subtler. And it was: the climaxes less brazen and the poetry more deeply felt. A captivating reading.

With so many British conductors - Gardiner, Norrington et al - building period-performance empires that grow if you blink, it's easy not to notice that much the same is happening in mainland Europe. There, for example, Phillippe Herreweghe seems every year to found another choir or orchestra tailored to some specific area of repertory. In France, Herreweghe is a household name; in Britain we can barely pronounce it. But his reputation on disc has soared recently, with some superb recordings for Harmonia Mundi; and he got a full house at the Barbican on Monday for a performance of Berlioz's L'Enfance du Christ that marshalled together two of his choirs, La Chapelle Royale and Collegium Vocale, and his specialist 19th-century band, the Orchestre des Champs-Elysees.

Personally I've never liked the limp anaemia of Berlioz's descriptive writing in this Sacred Trilogy: Herod's rage, the cabalistic dances of the soothsayers, the slaughter of the innocents ... they wouldn't ruffle a rouched curtain in the Boulevard Malesherbes. But for the work of a composer who thought big, L'Enfance has a modesty and restraint that can be touching; and Herreweghe observed it beautifully, with a lightness of touch that kept the textures open and clean, and solo voices (mostly French) to match. The dark but lean animation of Laurent Naouri's Herod was particularly pleasing; and so was the focused refinement of Paul Agnew as the Recitant. Later in the week, Herreweghe took the whole show into the recording studios for issue next year. If you like the piece, look out for it.

Finally, a note on the William Walton Trust which is gathering profile, these days, not just in the training of musicians but in finding them a platform for their work. On Wednesday it organised a concert at Stationers Hall, London, including the premiere of a string quartet written at the Trust's invitation and with the benefit of its hospitality - at Walton's home on Ischia in the Bay of Naples. The composer was a young Oxford academic, Hugh Collins Rice, the performers the Coull Quartet; and although it proved to be a piece more about the exploration of texture than the development of ideas, it certainly repaid the Trust's investment. It also made me want to hear the next piece the composer does. Always a good sign.