Tractate Yoma

The Masechet Yoma is the fifth treatise of the Seder Moed.
The Yoma gemara is the study of the work of the Kohen Gadol on Yom Kippur, as well as the laws related to fasting.
The Yoma tractate bears this name which means in Aramaic Today that is to say the special day. In other words: Yom Kippur.