And the 50th mitzvah is that we are commanded to bring an offering in addition [to the daily offerings1] during the holiday of Sukkos.

The source of this commandment is G‑d's statement2 (exalted be He), "You must offer a burnt-offering as an appealing fragrance to G‑d [13 bulls, two rams, 14 yearling sheep...and a sin-offering of one goat]."

This is the musaf haChag offering.