Halakhah for Taanit 53:4

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.