The classical music industry loves nothing more than an anniversary.

The bicentenary of Verdi's birth, 40 years since the founding of this choir or that - it's the backbone of festival programming, the go-to pretext for indulging in nerdy corners of the repertoire. Needless to say, some anniversaries are worth celebrating more than others. Peter Maxwell Davies, Scotland's greatest living composer, turned 80 earlier this week. Given that he was diagnosed with leukaemia last year and told he had six weeks to live, this is a birthday that deserves every one of the commemoration concerts it is getting, from Venice to Moscow to Kirkwall.

"Normally I would treat birthdays as an ordinary day," Maxwell Davies told me, typically matter-of-fact. But of course he hasn't got away with being so discreet this time. "There have been parties for my significant birthdays in latter years, and last year when I had been so ill and went home, the neighbours appeared with food and drink and that was very nice."

This year the festivities started early. Back in June at the St Magnus Festival - the Orkney midsummer series that Maxwell Davies helped set up shortly after he moved to the islands in the 1970s - young musicians gathered in a packed auditorium at Kirkwall Grammar School to perform his music for children. There have been performances in Italy of the composer's firebrand early works Eight Songs For A Mad King and Vesalii Icones, and the Proms in London have staged a minor retrospective of his chamber and orchestral programmes. On Sunday there is an afternoon of concerts in Glasgow featuring the Hebrides Ensemble, guitarist Sean Shibe and the Scottish Chamber Orchestra.

It's little wonder that Maxwell Davies isn't in the mood to talk much. "The whole day will be taken up with rehearsals, parties, the Proms, radio interviews and so on," he told me last week via email after he had to cancel two phone conversations because he was feeling low on energy.

He hasn't stopped working: he composed much of his Tenth Symphony in his hospital room last year, and before he left Orkney to travel to his various birthday concerts this summer he had just begun a new setting of texts by George Mackay Brown. In radio interviews and pre-concert talks his spark is unmistakable, but he is understandably protective over how much stamina he is willing to spend on the music industry.

The last time I saw him he was in his element. It was at that schools concert in Kirkwall, a tremendously fond affair full of appreciation for what he and the festival have contributed to Orkney over the past four decades. The composer watched the young performers with a delighted smile and afterwards bounded onto the stage to receive a cake and a round of Happy Birthday. I got the sense that there was nowhere he would rather be.

Before the concert he spoke passionately about writing music for children and the future of music education. "We're seeing the demise of school orchestras, and school choirs barely exist," he said, describing his feelings as "despairing". "When I wrote these pieces, I took it for granted that they'd still be part of the culture. But successive governments have cut and cut and cut." When a member of the audience asked what could be done, Maxwell Davies replied: "Apart from the fuss I've made about it all my life, I don't know what to do. I will continue to write educational music of the highest quality I can. And despite being Master of Queen's Music, I will continue to rail against the establishment."

Last week he re-emphasised the importance that his music for young people has played throughout his career. "Since working at Cirencester Grammar school so long ago, working with them [children] and writing for them has been part of my musical life," he wrote. "When I moved to Hoy and started the St Magnus Festival, I was able to write for local schools and have wonderful performances by local children of all ages under the inspirational leadership of Glenys Hughes - the Two Fiddlers, Kirkwall Shopping Songs and Cinderella to name but a few. Since moving to Sanday I have continued the tradition with the children's fiddle band."

It is this side of Maxwell Davies - the one who composes affectionate music for children, and stately royal wedding anthems as Master of the Queen's Music - that has eclipsed his former reputation as a musical troublemaker. "The role of Master of the Queen's Music was enjoyable for me," he wrote. "Her Majesty was immensely supportive and gave me a free hand. I took the job seriously, as I do all composing. I wrote them a carol every year, and a symphony (the Ninth) for the Queen for the Jubilee. Anything which seemed appropriate. All my experiences with the Palace were positive. I enjoyed it immensely."

These words are hardly imaginable from his fierce former self, who rebelled against his school and shocked the musical world with the very visceral anger of works like Eight Songs and St Thomas Wake. Over the decades the extreme edge of that anger has softened and been replaced by a willingness to work within the establishment, but remnants are still there.

On Sunday the SCO performs a piece Maxwell Davies wrote last year called Ebb Of Winter. It is a careful, gracious score that nearly ends in a major triad - except it doesn't. At the last minute, a single note of dissonance rings out defiant overtones, a neat little hint of rebellion.

Maxwell Davies will be in Glasgow on Sunday to speak at the Max At 80 celebrations, then he's heading home to Orkney to vote in the referendum. He has already stated that he'll be voting Yes: "Here we are, ruled by a bunch of Bullingdon Club millionaire Tories who don't understand anything about Scotland," he told Tom Service in an interview last month. Last week I asked whether he has enjoyed the spirit of the debate. "Well, the debate is not so much interesting as the outcome," he replied, "which may not please Scotland but could help towards more autonomy and eventual 'freedom'." The question for Orkney and Shetland, he added, "is possible autonomy for us. Not something talked about much or taken very seriously except in Orkney and Shetland!!"

But first things first. Earlier this year Maxwell Davies told me about "a very special bottle of Highland Park" he's had for so long he has "really no clue how old it is any more". Has he managed to keep it sealed for the big day? "The bottle of Highland Park will be consumed when I get home and with my neighbours who look after me so well," he wrote, signing off his email.

Max At 80 is at City Halls, Glasgow, on Sunday