Halakhah su Deuteronomio 18:8

And that is that He commanded us that the priests serve in watches, each watch serving one week. And the hand of all should not be mixed together (in service); except on festivals when all of the watches serve equally and anyone who comes from them can offer a sacrifice. And this has already been explained in Chronicles (I Chronicles 9:22) - how David and Shmuel divided them into twenty-four watches. And it is explained in Sukkah (Sukkah 55b) that the hand of all [priests] are equal on festivals. And the language of this commandment is His saying, "If a Levite would go, etc. he may come as he pleases. He may serve in the name of the Lord, his God [...] They shall eat equal shares" (Deuteronomy 18:6-8). And the language of the Sifrei (Sifrei Devarim 168:5, 169:3) is, "'He may come as he pleases.' I might think always. [Hence] we learn to say, 'from one of your gates' (Deuteronomy 18:6) - when all of Israel are assembled within one gate; and that is during the three festivals. I might think that all of the watches are [also] equal for offerings brought on the festival not for the sake of the festival. [Hence] we learn to say, 'aside from the sale of the fathers' (Deuteronomy 18:8) - what the fathers sold to each other, i.e., 'You [take them] on your Shabbat, and I on my Shabbat.'" This means their agreement about all of the watches of the service, one watch for each week. And so did Onkelos (Targum Onkelos on Deuteronomy 18:8), explain, "except for the allotment which comes on Shabbat, as our (fore)fathers have regulated." And they have already explained the laws of this commandment at the end of the Gemara in Sukkah. (See Parashat Shoftim; Mishneh Torah, Vessels of the Sanctuary and Those who Serve Therein 4.)

That the priests and the Levites work in the Temple in shifts: That the priests and the Levites work in the Temple in shifts (Sefer HaMitzvot LaRambam, Mitzvot Ase 36) - meaning to say, in [assigned] groupings - and not that the hand of all be involved together in the work; except for the holidays alone, when all would work together - all who would come would [take part] for the joy of the festival. And elucidated in the book of I Chronicles 24-26, is how David and Shmuel divided them, that they made twenty-four shifts of priests and twenty-four shifts of Levites, in order that each of their shifts could work two weeks a year. And in Tractate Sukkah 55b, they may their memory be blessed, said that on the festivals, the hand of everyone was equal. And about this is it stated (Deuteronomy 18:6-8), "If a Levite would go, etc." - and included in the Levite is a priest, since Levi was the father to all of the tribe - "and he will come in all the yearning of his soul. And he will serve in the name of the Lord, his God, like all his brothers, the Levites, who are standing there [...]. A portion like a portion shall they eat." And the language of Sifrei Devarim 168 is "'And he will come in all the yearning of his soul' - perhaps always" - meaning to say, even not on holidays. "[Hence] we learn to say, 'besides the sales to the fathers' (Deuteronomy 18:8): that which the fathers sold, this one to that one, 'you take on your week and I on my week'" - meaning to say, their agreement in the order of the workshifts, each week its shift. And so did the Translation [of Onkelos Deuteronomy 18:8] explain it, "except for the shift that comes on the Shabbat [of a regular week], as so did our fathers ordain."