Whence is it derived that all the priestly watches share equally in the (devoted portions of the) offerings of the festival? From (6) "then he shall come in all the desire of his soul to the place that the L-rd shall choose." I might think that the same holds true for all the days of the year; it is, therefore, written (6) "from one of your gates" — when all of Israel are assembled within one gate, i.e., during the three festivals.
(Devarim 18:7) "And he shall serve in the name of the L-rd his G-d, as all his brothers, the Levites, who stand there before the L-rd": He stands upon the floor. From here they ruled: If he sat or stood upon vessels or upon a beast or upon the feet of his fellow, (his service) is invalid.