Birchas cohanim is the blessing that the cohanim give to the people. Chazal instituted it into Shmone Esrei. However by mincha they said you should not include it as maybe the cohein ate and drank wine and cannot give the blessings.

Three times during the year the cohanim could bless the people 4 times in one day in Shacharis, Mussaf, Mincha, and Neilah [the closing of the gates like we pray on Yom Kippur]. These days are on a taanis [fast day], maamados, and Yom Kippur.

[Neilah was said on fast days]

Taanis 4:2

Maamados----It says in the posuk that the person must stand [be by it] on his karban. On public karbanos chazal decreed that some people from klal yisrael should stand on the korban, meaning not to work and to say certain tefilos.

There were 24 mishmaros [groups] of cohanim and leviim who went up to Yerushalayim each week to do their avoda and this group of maamados would go to their cities and say the pesukim of creation.