Halakhah su I Cronache 9:45

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.)