The tractate whose name means “the day” and refers to Yom Kippur is Yoma.
Yoma is the fifth tractate in Seder Moed, the Mishnah’s “Order of Festivals.” Its title comes from the Aramaic word yoma, meaning “the day,” an abbreviated reference to Yom Kippur, the Day of Atonement. The tractate is devoted chiefly to the Temple rites performed on that day.
Its eight chapters describe the preparation and work of the Kohen Gadol, or High Priest. Topics include his seclusion before Yom Kippur, the sacrifices, the incense offered in the Holy of Holies, the two goats, the scapegoat sent to Azazel, and the order of the day’s services. The final chapter turns to the five afflictions traditionally associated with Yom Kippur, including fasting and abstaining from bathing.
Yoma has both Babylonian and Jerusalem Talmudic Gemara. It is not the same as Berakhot, which focuses on blessings and prayer, Megillah, which centers on Purim, or Nedarim, which discusses vows.