And the 49th mitzvah is that we are commanded to perform the service of the fast day Yom Kippur — i.e. all the obligatory offerings and confessions — thereby achieving atonement for all transgressions, as stated in Scripture, i.e. all the laws written in the Torah portion Acharei Mos.1

The proof that the entire service counts as one single commandment is from the end of the 5th chapter of tractate Kippurim,2 "The entire service of Yom Kippur must be performed in the order written;3 if one action was performed out of order, everything is invalid."

All the details of this mitzvah have been explained in the tractate dealing with it exclusively, tractate Yoma.