The Piano Concerto #21 in C Major was composed in March, 1785. This was in the midst of a particularly productive period in Mozart's life, as his father Leopold reported in a letter to Mozart's sister Nannerl. "Every day there are concerts; and the whole time is given up to teaching, music, composing and so forth..."
The piece was first performed as part of a series of Lenten concerts - entertainments during the season before Easter frequently involved musical performances since other types of activities were forbidden during Lent. The occasion of the premiere was a "benefit concert at the National Court Theater" in Vienna.