Halakhah su Deuteronomio 18:6

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."