"And the Levite shall do the service of the Tent of Meeting"—Numbers 18:23.
The Levites – and only the Levites – are commanded to discharge the Levite duties in the Holy Temple. These duties include serving as gatemen and accompanying with song the offering of the sacrifices.
"Each man individually to his task and his load"—Numbers 4:19.
The kohanim (priests) and Levites each have their unique tasks in the Holy Temple. As such, Levites are forbidden to discharge any of the duties that are the domain of the kohanim, and vice versa.
Indeed, it is also forbidden for a priest or Levite to get involved in a task that was assigned to a fellow priest or Levite. The Talmud relates that Rabbi Joshua ben Chananiah once wished to aid his fellow Levite Rabbi Yochanan ben Gurgoda. "Desist from this," Rabbi Yochanan told him, "for you are already liable to pay with your life. For I am of the gatemen and you are of the singers..."
"You shall sanctify him, for he offers up the food offering of your G‑d; he shall be holy to you"—Leviticus 21:8.
We are commanded to honor and exalt the seed of Aaron, the kohanim (priests), and to acknowledge their holiness.
This mitzvah applies whether or not the specific priest desires such honor. This mitzvah applies also to maimed kohanim, though they are not eligible to serve in the Holy Temple.
We fulfill this mitzvah by honoring the priest with the first aliyah to the Torah and leading the Grace after Meals, and by allowing him to take the first choice portion of food.
Honoring the kohanim is, in effect, honoring G‑d, who chose them to serve Him [in the Holy Temple] and offer the sacrifices.
"And if a Levite comes . . . he may come whenever his soul desires . . . and he may serve in the name of G‑d, his G‑d, just like all his Levite brothers, who stand there before G‑d. They shall eat equal portions..."—Deuteronomy 18:6-8.
We are commanded to divide the kohanim (priests) into shifts, so that each week another shift should serve in the Holy Temple. Only during the festivals shall all the kohanim serve together.
The prophet Samuel and King David divided the kohanim into 24 shifts.
