On three junctures of the year the Kohanim raise their hands [to bless the people] four times during the day —- at Shacharis, Musaf, Minchah, and Ne'ilah —- on fast days, on maamados, and on Yom Kippur.

These are the maamados: Since the Torah states: Command the Children of Israel and say to them, “My sacrifice, my bread ...” Now, how can a person's sacrifice be offered when he is not present? The early prophets instituted twenty-four mishmaros. Corresponding to every single mishmar, there was a maamad in Jerusalem of Kohanim, Leviim, and Yisraelim. When the time approached for the mishmar to ascend, the Kohanim and the Leviim would ascend to Jerusalem, and the Yisraelim assigned to that mishmar would assemble in their cities, and read from the Story of Creation.