The Torah in Parashat Shoftim (Debarim 18:6-8) discusses the arrangement for the service of the Kohanim and Leviyim in the Bet Ha’mikdash. It first speaks of a Kohen or Levi who comes to the Bet Ha’mikdash wishing to serve, and command that he must be allowed to do so. But the Torah then commands abiding by “Mimkarav Al Ha’abot” – the “sales” made by the earlier generations. The Sages explain that this refers to the system that was later devised by the prophet Shemuel and King David, as we read in the Book of Dibreh Ha’yamim I (9:22-26), dividing the Kohanim and Leviyim into twenty-four “Mishmarot,” or shifts. Each shift would work for one week at a time, such that every shift would serve at least twice over the course of the year. This is considered a “sale” because each group of Kohanim “sold” its privileges to the other groups in exchange for its turn to serve. It thus seems that the Torah here first allows a Kohen to serve whenever he wishes, but then commands abiding by the rotation system that would be established. The Gemara explains that the first verse, which allows a Kohen to serve whenever he wishes, refers to the Regalim (the holidays of Pesach, Shabuot and Sukkot), when all the Kohanim were given an equal opportunity to serve. On these three holidays, it was “all hands on deck,” so-to-speak, as all the Kohanim had the opportunity to participate in the Aboda (service) in the Bet Ha’mikdash. The rest of the time, however, only the Kohanim assigned to each week’s shift would serve during that week.
The Sefer Ha’hinuch explains that the Torah required following this system in order to ensure that the various responsibilities would be properly fulfilled. If a job is assigned to a large group of people, then they might likely all assume that the job will be done by others, and it will thus be left undone. Or, they might argue with one another over who tends to which aspects of the task. If, however, a job is assigned to a small group, they will likely work effectively, cooperating with one another. The Torah therefore commanded dividing the Kohanim into small groups to ensure that the various functions of the Bet Ha’mikdash will be tended to in an efficient and peaceful manner.
Each of the twenty-four Mishmarot had a leader, who was called the “Rosh Ha’mishmar,” who divided the shift into seven groups, each of which served one day of the week assigned to that Mishmar. Each of these seven groups was called a “Bet Ab,” and the head of the “Bet Ab” would divide the day’s responsibilities among the Kohanim in that group. The Mishmarot changed on Shabbat.
The prophets also instituted the “Ma’amadot” – twenty-four groups of Yisraelim corresponding to the twenty-four shifts of Kohanim. The members of each “Ma’amad” (“Ansheh Ma’amad”) would represent the Jewish People in Jerusalem for the week of their shift. As the sacrifices were offered on behalf of the entire nation, it was only appropriate for members of the nation to be present in Jerusalem when the sacrifices were being offered. The “Ansheh Ma’amad” would fast each day from Monday through Thursday, and recite special prayers in a synagogue in Jerusalem.
A Kohen who needed to bring a personal sacrifice was allowed, if he so wished, to perform the service for his sacrifice, even if that week was not his turn to serve in the Bet Ha’mikdash.
The Sefer Ha’hinuch writes that one violates this prohibition by not allowing a Kohen to go to the Bet Ha’mikdash on one of the Regalim, when all Kohanim are allowed to participate in the Aboda. Interestingly, the Sefer Ha’hinuch does not write that a Kohen violates this prohibition by failing to go to serve on the week of his Mishmar. The reason, perhaps, is that the Sefer Ha’hinuch felt that the obligation is to arrange the system, and so once the system was arranged and put into place, the Misva cannot then be violated. It is only the other aspect of this Misva that can be violated – granting permission to all Kohanim to serve on the Regalim. This part of the Misva is transgressed by denying a Kohen this opportunity.