From Wikipedia:
Symphony No. 7 in C major, Op. 105, was the final published symphony of the Finnish composer, Jean Sibelius. Completed in 1924, Symphony No. 7 is notable for being a one-movement symphony, in contrast to the standard symphonic formula of four movements…
The form of the symphony is startlingly original. Since the time of Joseph Haydn, a movement in a symphony would typically be unified by an approximately constant tempo and would attain variety by use of contrasting themes in different keys. Sibelius turned this scheme on its head. The symphony is unified by the key of C (every significant passage in the work is in C major or C minor), and variety is achieved by an almost constantly changing tempo, as well as by contrasts of mode, articulation and texture….
Although the symphony apparently first existed in embryonic form in D major, it eventually attained the home key of C major. There was a time when composing in C was considered fruitless—it had “nothing more to offer”. But in response to this symphony, the British composer Ralph Vaughan Williams said that only Sibelius could make C major sound completely fresh. Peter Franklin, writing of the Seventh in the Segerstam–Chandos cycle of Sibelius symphonies, calls the dramatic conclusion “the grandest celebration of C major there ever was.”
An interesting factoid that may serve to contrast the Aryan vs. Semitic aesthetic sensibility:
[Sibelius’] later works are remarkable for their sense of unbroken development, progressing by means of thematic permutations and derivations. The completeness and organic feel of this synthesis has prompted some to suggest that Sibelius began his works with a finished statement and worked backwards, although analyses showing these predominantly three- and four-note cells and melodic fragments as they are developed and expanded into the larger “themes” effectively prove the opposite.
This self-contained structure stood in stark contrast to the symphonic style of Gustav Mahler, Sibelius’s primary rival in symphonic composition. While thematic variation played a major role in the works of both composers, Mahler’s style made use of disjunct, abruptly changing and contrasting themes, while Sibelius sought to slowly transform thematic elements. In November 1907 Mahler undertook a conducting tour of Finland, and the two composers were able to take a lengthy walk together, leading Sibelius to comment:
I said that I admired [the symphony’s] severity of style and the profound logic that created an inner connection between all the motifs … Mahler’s opinion was just the reverse. “No, a symphony must be like the world. It must embrace everything.”