Taanis 4:1-2

Taanis 4:1

On three times of the year, kohanim would duchen four times a day, in the shacharis, musaf, mincha and neilah prayers: on fast days, on maamados (which will be explained starting with the next mishna) and on Yom Kippur.

Taanis 4:2

The ‏maamados are as follows: Numbers 28:2 says, "Command the children of Israel and say to them, My offering, My bread...." How can an offering be brought for someone if he is not standing near it at the time? (That is, the entire Jewish people could not stand by as the public sacrifices were being offered.) Because of this, the early prophets instituted divisions, and for each one there was a "maamad" of kohanim, Levites and Israelites in Jerusalem. When each division's turn arrived, the kohanim and Levites went to Jerusalem and the Israelites of that division assembled in their cities to read the creation account (to indicate the connection between the Temple service and the endurance of the world).