Tractate Yoma belongs to Moed, the Mishnah’s Order of Festivals. Its subject is chiefly the ritual and laws of Yom Kippur, the Day of Atonement.
Moed is one of the six major divisions, or orders, of the Mishnah. It gathers tractates dealing with sacred times, including Shabbat, Passover, Sukkot, Rosh Hashanah, and Yom Kippur. Yoma is the fifth tractate in this order, following Shekalim and preceding Sukkah.
The classification reflects Yoma’s focus on a calendar observance rather than on sacrifices in general. That can cause a common confusion: Kodashim is the order devoted broadly to holy things and sacrificial offerings, but Yoma is placed in Moed because it explains the observance of a specific festival day. The tractate later became a major source for understanding the High Priest’s Temple service.