We continue with chapter 4 of Maseches Taanis, mishnah 2. The previous mishnah mentioned the institution of the maamados, and now our mishnah comes to explain it and asks: "Eilu hein maamados?" - what are those maamados that were mentioned above?
The source of the matter - "Es korbani lachmi":
"Lefi shene'emar: Tzav es Bnei Yisrael ve'amarta aleihem es korbani lachmi" - in the passage dealing with the daily tamid offering, which was brought every day, Moshe was commanded to speak to Bnei Yisrael and tell them that they must bring this offering. In other words, this offering is the obligation of Bnei Yisrael, and they are its owners.
"Veheiach korbano shel adam kareiv":
From here the mishnah asks: "Veheiach korbano shel adam kareiv vehu eino omeid al gabav?" - how can it be that a person's offering is brought while he is not standing there over it?
The enactment of the early prophets:
Because of the need for the owners to be present at the time the offering is brought, "hiskinu nevi'im harishonim esrim ve'arba mishmaros" - the early prophets instituted twenty-four watches, the twenty-four groups into which the kohanim and the Levi'im were divided.
"Al kol mishmar umishmar hayah maamad biYerushalayim" - a maamad of kohanim, of Levi'im and of Yisraelim. Not only the kohanim and the Levi'im were divided into watches; the Yisraelim too were divided into groups that went up to Jerusalem.
"Higia zman hamishmar la'alos - kohanim uLevi'im olim Yerushalayim" - when the turn of that watch arrived, the kohanim and the Levi'im would go up to Jerusalem, and Yisraelim from that maamad would accompany them.
The Yisraelim who remained in their cities:
The mishnah continues and discusses those Yisraelim who did not go up to Jerusalem, and who were nevertheless part of the process: "VeYisrael shebe'oso mishmar miskansin le'areihen vekorin bemaaseh bereishis" - they would gather in their cities and read in the Torah from the passages of the account of Creation.
This reading, as stated, was done by the group that did not go up to Jerusalem; however, most of the commentators understand that the group of Yisraelim who did go up to Jerusalem also devoted their day to prayer and to reading the account of Creation, just like their brethren who remained in their cities.
In summary: in this mishnah we learned the root of the maamados: from the command "Tzav es Bnei Yisrael ve'amarta aleihem es korbani lachmi" it emerges that the tamid offering belongs to all of Israel, and hence the question of how a person's offering can be brought without his standing over it. Therefore the early prophets instituted twenty-four watches, and corresponding to them maamados of kohanim, Levi'im and Yisraelim - these go up to Jerusalem, and those gather in their cities and read the account of Creation.