Taanit, Chapter Four, Mishnah One
During regular weekday prayers, the priests offer a priestly benediction as part of the Amidah. This consists of their lifting up their hands and reciting Numbers 6:24-26. The priestly benediction is normally done only during Shacharit, the morning service and not at Minhah. On Shabbatot and holidays that have a Mussaf service, it is also done during Mussaf. It is not normally done during Minhah because it is feared that the kohen may have drunk some wine during the day and it is forbidden to recite the priestly blessing while drunk. Such a concern does not exist on fast days, when the priests would not be drinking wine.
The mishnah also teaches that on fast days another service is added, Neilah, which means closing [of the gates]. This service is added as a special additional supplication.
On three occasions during the year, on fast days, on maamadot, and on Yom Kippur the priests lift up their hands to bless [the people] four times during the day–at Shaharit, at Mussaf, at Minhah and at Neilah.
Maamadot is an institution that was mentioned in passing above in 2:7 and will be explained in greater detail in tomorrows mishnah.
On these three types of days the priests offer the priestly blessing every time there is a prayer service. According to the Babylonian Talmud, the mishnah is not entirely accurate since there is no Mussaf service on fast days or on maamadot. What the mishnah means to say is that every time there is a prayer service, the priests lift up their handsfour times on Yom Kippur and three times on the other occasions. However, other sources seem to hold that there is a Mussaf service on fast days and on maamadot.