A day of free concerts celebrated Lucerne's 75th birthday, while Abbado's spiritual take on Schubert and Bruckner was compelling and deeply affecting

Strange things happened at the Lucerne Festival over the weekend: this bastion of all that's most privately-sponsored and frankly expensive in orchestral culture (tickets for the big-name concerts in Lucerne can cost about 50 times as much as a Promming place...) opened its doors on Sunday for its 75th birthday celebrations, a day in which every note played - from Messiaen's Turangalîla Symphony to Beethoven's Fifth, from German percussion star Martin Grubinger's Percussive Planet to live remixes by Found Sound Nation - was free for everyone in the city.

Everything was packed, all day. It helps that the Festival is the location of two of the most visionary projects in classical music in recent years: Claudio Abbado's Lucerne Festival Orchestra, and the Festival's International Academyfor young musicians, founded by Pierre Boulez with the aim of transplanting the music of the 20th and 21st centuries into the bloodstream of the next generation of performers. Both ensembles were at the heart of the day's concerts: David Robertson conducted the Academy's orchestra in an infectiously cheery performance of Steve Reich'sThe Desert Music, followed an hour later by the Turangalîla Symphony, played with overwhelming technical conviction but with less than the emotional extremity the piece demands. In fluent German, Robertson interrupted the performance halfway through. Sensing that some Lucerners might be new to Messiaen's orgiastic sound world, he urged them to stay because the final movement of Turangalîla "does things that are unique in music history". He was right, too, and the vast majority of Messiaen newbies stayed to the rapturous conclusion. There was no stinting on the new music elsewhere on the programme: Stockhausen's Tierkreis in an ambitious show for kids, string quartets by Cage and Georg Friedrich Haas from the American Jack Quartet.

There was a rumour that Abbado might conduct the day's concluding concert from the Lucerne Festival Orchestra of Mozart - a voluptuously virtuosic Flute and Harp Concerto with soloists Jacques Zoon and Letizia Belmondo - but in the end, Rossini's Italian Girl in Algiers overture was conducted by his assistant, Gustavo Gimeno. Even though the orchestra's guiding conductorial spirit failed to materialise, that didn't take away from a day that showcased the best and most adventurous achievements of Lucerne's recent history, completely for free: something that UK festivals could surely learn from - if they could afford it…

There could have been another reason for Abbado's absence. The night before, he conducted his orchestra in a programme of two of the greatest unfinished symphonies, the two movements of Schubert's 8th and the three ofBruckner's 9th. I've been lucky enough to hear Abbado's relationship with these musicians develop over the last few years (above all in the finest and most transformative Mahler symphony cycle that I've ever experienced), but even the depths and heights the orchestra have traversed in their music-making over the last decade didn't quite prepare me for this Schubert and Bruckner.

Abbado has always been the most extreme conductor working today in terms of the freedom with which he trusts his musicians and the economy of his gestures. But he took all that to new levels in this concert. He eliminated absolutely everything from his body language that wasn't directly to do with the expression and progress of the spiritual experience of these pieces. There was hardly a downbeat worthy of the name in the entire concert, and there were no grand gestures of rhythm or pulse. Instead, there was only a distilled, gossamer communication of the music's line and internal motion to his musicians. This was genuinely spiritual music-making: in the ethereal beauty of the sounds this orchestra created, in the extraordinarily broad but flexible speeds Abbado chose in both pieces, and even more fundamentally, in the sense that Abbado was drawing all of us into an intense interior journey that he somehow managed to convert into sonic, symphonic energy.

It was sometimes difficult, even painful, to witness this progress of unadulterated musical spirit. Every note of both symphonies was staged somewhere beyond: removed from the conventions of how they are usually performed, set in a different place from the public realm they most often inhabit. There was no comforting worldliness in Abbado's vision of these pieces: the Schubert was unmoored by his slow tempi and by the gaspingly quiet dynamics this gigantic orchestra produced, which even made the usually gloriously tuneful slow second movement glow with a disturbing, dark light.

The Bruckner was still more disembodied. That wasn't just a question of slowness, it was more that the whole piece levitated on the hyperreal musical beauty and subtlety that Abbado asked from his players. The performance hovered between the world of reality - the concert hall, the audience, the lights, the physicality of the performers - and some other place, some other state of being, that we were invited to enter into. And to go with the music was to discover a terrifying darkness as well as radiant light, above all in the apocalyptic dissonance that the third movement climaxes with. And then? Only the question mark that comes after all performances of this symphony that end with the adagio. Abbado and his orchestra's performance was complete in its intensity, but devastating in its essential incompleteness: having glimpsed this place between worlds, we and the musicians had to wrench ourselves back to reality after the silence at the end of the concert collapsed into applause. Abbado seemed existentially exhausted at the end of the performance. How could he not have been? And yet, in his 80th birthday year, to confront and to realise so compellingly these visions of the beyond takes a special kind of musical and moral courage.