100:1 It’s a positive commandment from the Torah for the kohanim to bless the people (birkas kohanim, AKA “duchening”), as per Numbers 6:23, “So shall you bless the children of Israel.” If a kohein doesn’t have any of the disqualifying conditions but he doesn’t go up to bless the people, he violates a positive mitzvah. There is no violation unless he was called, as per Numbers 6:22, “say to them.” Onkelos explains this to mean, “like this you shall say to them.” (Being told individually to go duchen and the shaliach tzibbur calling out “Kohanim!” both count as being called – Mishnah Brurah 128:10.) The current practice (outside of Israel) is for the kohanim only to bless the people on yom tov, since then we are full of the joy; on Yom Kippur there is joy because of the forgiveness we receive. Because of this, the kohanim are able to bless us with a full heart, which is not the case on other days, even on Shabbos during the year, because they are troubled by worries about their income and the loss of a day’s wages. (The Mishnah Brurah objects to the implication that the kohanim are troubled by not being able to work on Shabbos. He feels the explanation has been garbled – MB 128:167.) Even on yom tov, they only bless the people at musaf (the additional prayer service), after which we leave shul rejoicing with the joy of the festival. It is also our practice not to duchen when yom tov falls on Shabbos, except for Yom Kippur. We duchen even when Yom Kippur falls on Shabbos. There are places that have the custom to duchen on every yom tov, even if it falls on Shabbos, and this is more correct.
100:2 We only duchen with a minyan of ten. This number includes the kohanim because the kohanim are also included in the blessing as the Torah tells us (Numbers 6:27), “I (G-d) will bless them.”