It's half a century to the day that Jean Sibelius, Finland's greatest composer, and a towering national figure, died.

So it's appropriate that tonight sees the first of three concerts over the coming week in which the CBSO performs all seven of his symphonies under Finnish music director Sakari Oramo. As well as nodding to the anniversary, the complete cycle launches Oramo's final season as music director.

What is perhaps more surprising is to discover that the Sibelius anniversary is being given a low-key treatment in Finland.

"In Finland I have made a point of not making a big thing of it with my orchestra [the Finnish Radio Symphony]," says Oramo. "I knew we would be doing this here, so we're just keeping completely clear of it."

Does he think we in Britain are generally more obsessed with anniversaries than our European neighbours? " I would say so," he agrees.

So how did he go about programming the symphonies over the three concerts?

"You can do it in various ways, but I think if you add some small pieces like we're doing it adds to the general appeal of the programming, rather than if you only do 1,2,3,4,5,6, 7."

Tonight's first concert includes the Fourth and First symphonies, naturally ending with the larger, more romantic First.

The more compact Fourth - given its UK premiere, with the composer conducting, in Birmingham Town Hall in 1912 - is included in the first half, and is sandwiched between the popular Swan of Tuonela and another Sibelius piece receiving a much-delayed UK premiere, the Three Symphonic Pieces for Cello and Orchestra.

"I think it makes perfect sense to do The Swan of Tuonela in front of the Fourth symphony, although they are quite far apart in time, because it is also very dark. And the cello pieces are very much in the period of the Fourth Symphony."

These pieces, played tonight by Finish cellist Martti Rousi, will be a complete mystery to most Sibelius fans.

"It's a compilation, actually, it was never intended by Sibelius to be a complete work," Oramo explains. "Two of the pieces were meant to be played in churches and it's quite rare because Sibelius was completely atheistic, but I think he probably had an urge to improve his public appeal at the time by having music that could be played in church.

"But they are quite amazing little pieces. They are maybe music in the world of the Sixth Symphony, with lots of polyphonic writing.

"The third piece, Malinconia, was written for cello and piano and was only orchestrated last year. It's so dark and meditative in a funny way. It has a completely unique atmosphere."

Sibelius is one of the most instantly recognisable of composers, yet much of his output (the Swedish record label BIS has recently announced it intends to record every note he wrote) is very little known. I suggest there are probably many small pieces which point towards different stylistic rooms.

"Yes, he had lots of rooms-off, and of course he wrote this music mostly for money. There are lots of little songs, lots of piano pieces, which you almost never hear, outside Finland at least. Lots of little chamber pieces, too.

"I think he was very good at capturing a mood, so he was also a good miniaturist, though we think of him primarily as a symphonist. I think this was also a counterpoint to his very hard-working way of writing the bigger works. He could turn round two little pieces in a day, and it was therapy as well, in a sense."

The Fourth Symphony, written at a time when Sibelius was suffering from throat cancer, is his most intense. He described it as "by no means a concert item" and Frederick Delius, hearing the rehearsal in Birmingham, declared: "Dammit - this is not conventional music!"

It retains a reputation as something of a hard nut, but Oramo says: "I don't quite see it in that way. It's probably the best composed of them all, but its very dark atmosphere makes it less appealing to large audiences."

The CBSO has had a remarkable relationship with the Sibelius symphonies over the last 25 years, having recorded complete cycles with both Oramo and his predecessor Simon Rattle.

Many of Oramo's concerts in this final season as music director (he steps down to become chief guest conductor next year, when he becomes principal conductor of the Royal Stockholm Philharmonic), revisit key works he and the orchestra have performed together over the last decade.

These include Elgar's Enigma Variations , Holst's The Planets and Mahler's Fifth Symphony. For his two farewell concerts next June he is coupling Janacek's Sinfonietta with Beethoven's Choral Symphony.

"Its mostly revisiting things," he says. "It's very much an exercise to round up what we've gone through in the last few years and to make pieces of music happen again that have been important during those years.

But there are also two further concerts in the IgorFest project to perform the complete works of Stravinsky. The most substantial rarity is the 56-minute choral work Persé-phone.

"It's very rare and it looks quite scary, but it's actually a beautiful piece of neo-classical writing, so it's actually not scary at all."

He doesn't yet know what he will be doing in his new role as chief guest conductor: "It depends so much on my future successor, what he or she wants to do, because of course I need to bring a counterbalance to that. But I would like to be able to continue with IgorFest for at least one more season and also it's important to do something with the chorus again."

On the question of his successor, he hints tantalisingly that an announcement may now not be too far away.

"I can say a corner has been turned. I don't want to be the one who leaks it, so I will keep my lips sealed. It's exciting!"

Sakari Oramo conducts the CBSO music by Sibelius tonight (7.30pm), Saturday (7pm) and next Wednesday (7.30pm) at Symphony Hall (Box office: 0121 780 3333).