“And when a Levite comes… and he shall come with all the desire of his heart to the place which Hashem will choose!” (Devarim 18:6)
It is a positive commandment that the Priests and Levites should serve in the temple in “watches”. This means that they should be divided into separate groups so that not everyone is occupying themselves with the holy service simultaneously. The only exception to this is on the festivals when everyone performed the service together because of the happiness of the festival. In the book of Divrei Hayamim (Part one 24:26) it is described how King David and the prophet Shmuel divided them, when they created twenty-four groups of Priests and twenty-four groups of Levites so that each group would serve two weeks each year. However, a Priest who has an offering of his own, may come whenever he wants and perform the service for his own offering.
Amongst the roots of this commandment are that since we know that if the responsibility for the service is shouldered by a small group of people it will be performed correctly without laziness, despair or argument, whereas a responsibility shouldered by everyone without specific people taking responsibility, will end up with one set of people giving the job to others and sometimes they will get upset with each other. During the festivals, however, because of the rejoicing they were commanded that everyone should be equal.
Amongst the laws of this commandment are that in each group there was one person who was appointed to be the head of the group and he would divide the group into smaller groups each of which would be responsible for one day. These smaller groups would also have heads who would give out the services and give jobs to each man. Every Shabbos they would change the groups and there was a rotation for whose turn it would be each week.
The earlier prophets decreed that there would also be twenty-four groups from the rest of Klal Yisrael made up of righteous G-d fearing men who would be called the “Anshei Ma’amad”. These would be the representatives of Klal Yisrael to stand over the public offerings offered on behalf of the entire Klal Yisrael, as Chazal said “And is it possible that the offering of a person should be brought up without him standing over it?” The members of the ma’amad who lived near Yerushalayim would enter the temple. All of them, whether they lived in Yerushalayim or other places, would gather in the synagogues and pray abundantly, and would fast on the Monday, Tuesday, Wednesday and Thursday of their designated week.
This commandment applies to males of the Priests and Levites when the temple was standing. One who does not allow his friend to serve when he wants during the festival has transgressed this commandment.