הלכה על תענית 53:2

From the laws of the commandment [is] that which they, may their memory be blessed, said (Mishnah Shekalim 5:1), "There were fifteen appointees in the Temple: one was over the times," meaning to say to determine the time of the sacrifice. And he immediately envigorates them and says, "Priests, stand at your services; Levites, to the platform; and Israel, to serve!" And once they hear his voice, each one comes to his work. And specific priests and Levites would serve there. And it is like they, may their memory be blessed, said that Moshe and Shmuel the Seer and King David divided them into twenty four shifts, as it is found in Taanit 27a. And each shift would serve for a week and take turns in a cycle. And the head of the shift would divide all the men of the shift into clans. And each day of the week specific men would work, and the heads of the clans would divide specific men, each one to his service, and they would not be allowed to assist one another [in their tasks]. And so [too,] the Levites, the singer would not be allowed to help the gatekeeper; nor the gatekeeper, the singer. And of these fifteen appointees that we mentioned, we have already said that the first was appointed over the times; the second was appointed over the closing of the gates; third over the guards; fourth over the singers, etc., as it comes in the Mishnah. [This] and the rest of its details are in [various] places in the Order of Kedoshim and in Sifrei and in Mekhilta, as we have said above (see Mishneh Torah, Laws of Vessels of the Sanctuary and Those who Serve Therein 3).

And the early prophets established (Mishneh Torah, Laws of Vessels of the Sanctuary and Those who Serve Therein 6:1-3) that twenty-four shifts of proper and sin-fearing men from the Israelites likewise be appointed. And in every place in the Talmud, they are called the men of [the] watch (anshei maamad); meaning to say they are the agents of Israel, to stand over the communal sacrifices, and like the matter that they, may their memory be blessed, said (Taanit 27a), "Is it possible for the sacrifice of a man to be offered and he not stand over it?" And for every [such] watch, one great [man] would be over them all, and he is called the head of the watch. And this was the custom of the men of the watch: On each and every week, they would gather [and] whoever of them was in Jerusalem or around it, would enter the Temple. But whether in Jerusalem or in the other places, they would gather in the synagogue, pray much, and fast on Monday, Tuesday, Wednesday and Thursday of that week. And the rest of their matters in prayer and in the reading of the Torah is like it is explained in Tractate Taanit and Megillah 22b. And they, may their memory be blessed, said (Sukkah 55a) that the hand of the shifts of priests and Levites was not equal in everything on the festivals, but rather only in the festival sacrifices, the distribution of the showbread, and the distribution of the two breads of [Shavuot]. But only the shift the time of which is fixed [for then] offers the vows, the oaths and the daily sacrifices, even on the festivals. [That] is to say, "A portion like a portion shall they eat" in the communal sacrifices; but they are not "A portion like a portion" in other things that the fathers already divided and fixed them [for] each and every shift on its week. And a priest (Mishneh Torah, Laws of Vessels of the Sanctuary and Those who Serve Therein 4:7) that had a sacrifice - behold, he could come to the Temple and offer it at any time he wanted, as it is stated, "and he will come in all the yearning of his soul"; meaning to say he can come to offer a sacrifice that is his at any time that he wants. And the skin is his. [These] and the rest of the details of the commandment are in Tractate Taanit and Megillah and at the end of Sukkah.