On the piano, the C major scale can be played by playing the white keys starting on C.
Twenty of Joseph Haydn's 104 symphonies are in C major, making it his second most-used key, second only to D major. Of the 134 symphonies mistakenly attributed to Haydn that H. C. Robbins Landon lists in his catalog, 33 are in C major, more than any other key. Before the invention of the valves, Haydn did not write trumpet and timpani parts in his symphonies, except those in C major. Landon writes that it wasn't "until 1774 that Haydn uses trumpets and timpani in a key other than C major... and then only sparingly." Most of Haydn's symphonies in C major are labelled "festive" and are of a primarily celebratory mood. (See also List of symphonies in C major.)
Many masses and settings of Te Deum in the Classical era were in C major. Mozart wrote most of his masses in C major and so did Haydn.
^H. C. Robbins Landon, The Symphonies of Joseph Haydn. London: Universal Edition & Rockliff (1955): 227. "In the course of composing his first symphonies, the tonality of C major became indelibly impressed on Haydn's mind as the key of pomp, the key of C alto horns, trumpets and timpani, the vehicle for composing brilliant and festive music, although at least during this period [the 1760s] he did not always reserve the tonality of C major for this particular kind of symphony: Nos. 2, 7 and 9, and possibly Nos. 25 and 30 ... are C major symphonies without the psychological manifestations inherent in most of the later works in this key. For the rest, however, the C major path is astonishingly clear; it can be traced from its inception, in Nos. 20, 32 and 37, through No. 33 and the more mature Nos. 38 and 41 to its synthesis in the Maria Theresia (No. 48) and No. 56. It continues with No. 50 and proceeds through Nos. 60, 63, 69, 82 and 90, reaching its final culmination in No. 97."
^James Webster & Georg Feder, The New Grove Haydn. New York: Macmillan (2002): 55. "The Missa in tempora belli ... in C features the bright, trumpet-dominated sound typical of masses in this key."
^Philip Coad, "Sibelius" in A Guide to the Symphony edited by Robert Layton. Oxford University Press. Sibelius's Seventh "is in C major, and a look back at the previous four symphonies [by Sibelius] will reveal how great the domination of C major has been [in his music]. It is the key of the Third, the relative major of the Fourth and the important 'neutral agent' in its Finale, the key which first forces away the tonic in the Fifth's Finale, and the principal opposition – the key of the brass – in the Sixth. Although it is now the tonic key, C major is also strongly associated with brass in the Seventh Symphony."