Massekhet Yoma is a chronological presentation of the various activities that take place in preparation for the Temple service on Yom Kippur, beginning with those things that need to be done prior to the holiday, and culminating with the day itself.
The first Mishna in the massekhet teaches how the Kohen Gadol is taken aside for an entire week of preparation and purification in anticipation of Yom Kippur. Since the entire complicated service will be done by him, reaching a climax with his entering the Holy of Holies – it was clear that serious preparation was essential. The Sages of the Talmud learned this from the Torah’s description of the Tabernacle in the desert, where Aaron and his sons were confined for a week during the ceremonies inaugurating the Mishkan to practice the service that needed to be done afterwards.
This preparation became even more important during the Second Temple period. Although the ideal Kohen Gadol should have been a scholar and righteous person, for a variety of reasons the person who filled the position during that period often did not live up to that expectation. This led the Sages to institute rules that would ensure that the High Priest would be knowledgeable in the service that he was to perform, and that he would do it correctly.
Aside from the day of Yom Kippur, all of the kohanim have the opportunity, and, in fact, were required, to take a turn in the Temple service. Our Gemara asks whether every beit av – family of priests – should be required to spend a week preparing for their turn working in the Beit ha-Mikdash, a suggestion ultimately rejected by the Gemara.
The idea of patrilineal priestly families – of a beit av – stems from a very early division of the kohanim – as early as the time of King David (see I Divrei haYamim 24:1-18) – into 24 mishmarot (watches). The same number existed during the Second Temple, as well, although it was a new division of labor, since only four priestly families returned to serve in the Second Temple. Each of the 24 “watches” was divided into six families (beit av). Every “watch” would go up to Jerusalem to work for one week at a time, so that in the course of a year each “watch” would work approximately two weeks. During the festivals of Pesah, Shavu’ot and Sukkot all the kohanim would come to work together.
During the week that a given mishmar was in the Temple, each beit av would work on a specific day, and only if there was an inordinate amount of work would a second family join them. Thus, generally speaking, every family of kohanim would work on two set days during the year.