Taanit, Chapter Four, Mishnah Two
This mishnah explains what the mamadot were and their origins.
What are the maamadot? Since it is said, Command the children of Israel and say to them: My offering, My food (Numbers 28:2). Now how can a mans offering be offered and he is not present? [Therefore] the former prophets instituted twenty-four mishmarot (guards). For each mishmar there was a maamad [at the Temple] in Jerusalem consisting of priests, Levites and Israelites.
When the time came for the mishmar to go up [to Jerusalem] the priests and Levites went up to Jerusalem and the Israelites of that mishmar assembled in their cities and read the story of creation.
Numbers 28:2 states, Command the Children of Israel saying: Of my near-offering, my food, as my fire-offerings, my soothing savor, you are to be in charge, bringing it near to me at its appointed time (this translation is from Everett Fox, who translates very literally.) This verse is said in reference to the tamid offering. The verse seems to imply that each Israelite is to offer the tamid and yet it is obviously impossible for all of Israel to be personally responsible for one offering. The maamadot are a means through which all of Israel is able participate in the tamid, the one offering that is offered twice every day.
The maamadot correspond to the mishmarot, the twenty-four groups of priests each of which serves one week in the Temple. It is not entirely clear what made up the maamad. According to one interpretation the maamad a group of priests and Levites who were not serving in the Temple together with some Israelites who would go to Jerusalem. According to another interpretation the priests and Levites were part of the mishmar and the Israelites were part of the maamad.
When time came for the mishmar to go to Jerusalem, the priests and Levites would go to Jerusalem and the Israelites from that mishmar, those who were not part of the maamad, would gather together in their cities and begin to read the story of the creation of the world, as we shall learn tomorrow.
We should note just how foundational this institution may have been in rabbinic thought. The sacrificial service is the most elitist element in Judaismin order to participate one must be a kohen; even Levites can only partially participate. Since one can only be born a kohen, there is no way for most of Israel to participate in this most central aspect of Judaism. By instituting the maamadot the rabbis seem to have found a way to make Judaism far more egalitarian. While it is still true that Israelites are limited as to what they can do, they are allowed to take part in this type of worship of God and it seems that their participation is not considered less significant than that of the kohanim themselves.