Why do kohanim bless every day in Israel but only on holidays in many shuls outside it?
In the Land of Israel, the custom is for kohanim to go up and bless the congregation every day. In the lands the Rema wrote about, they went up only on a holiday, and only at Musaf. On an ordinary weekday there, the leader says this paragraph instead. That is why, in shuls outside Israel that keep the Rema's custom, a weekday prayer has it.
The Rema gives a reason about mood. The one who blesses should have a full heart, and on a holiday people are in a festive mood. On other days, he says, they are busy with thoughts about earning a living and about work they are missing.
The Mishnah Berurah (a central commentary on daily Jewish law, written around 1900) says that in the Land of Israel and in Egypt the custom is to bless every day, and that the halachic authorities praised that custom. He also says the Rema's sentence is worded awkwardly, since holy people would not grieve over lost work on Shabbat. His reading: on Shabbat the problem is minds busy with livelihood, and on weekdays it is the work they would be missing.
So this is a difference in custom, and the sources say so openly. The Rema explains the practice of his lands. The Mishnah Berurah reports a different practice and says it was praised.
What stays the same is the gap this paragraph fills. The Rema says that at every morning prayer and Musaf where the kohanim do not bless, the leader says it.
Shulchan Aruch, Orach Chaim 128:44, Rema,Mishnah Berurah 128:164 and 128:167If no kohen is there, is the leader's paragraph a real blessing?
The sources say it is a request, not the blessing itself. The Mishnah Berurah explains: Amen only fits when you answer a kohen who is blessing. The leader is only asking that God bless us with the blessing the kohanim give.
That is why the Shulchan Aruch says the congregation does not answer Amen. They say "ken yehi ratzon," which means "so may it be Your will." The Mishnah Berurah says to say it after the third verse, because the whole paragraph is one matter. He adds that some answer after each verse.
The paragraph is also a later layer. Tosafot bring a view that in Gemara times the kohanim were called right after Modim with nothing in between. That view says the paragraph came in later, and it came as a prayer.
One more sign that it is custom-level: the Mishnah Berurah says that if a leader forgot to say it, we do not make him go back.
Shulchan Aruch, Orach Chaim 127:2,Mishnah Berurah 127:7, 127:10 and 127:11,Tosafot, Berakhot 34aWhy are we told not to look at the kohanim when they bless?
Where kohanim do bless, the Shulchan Aruch says the people face them and focus on the blessing, but do not look at them. The kohanim keep their own eyes down, as in prayer. The Rema adds that the kohanim let the tallit hang over their faces, so they do not look at their own hands. Their hands are usually outside it. In some places they are kept inside, so the people will not look at them.
The Gemara in Chagigah gives a stern reason: whoever looks at a rainbow, at a Nasi (the head of the Jewish people), or at the kohanim will have dim eyes. But for the kohanim, it specifies a time: when the Temple stood, and they blessed on the platform with God's Name as written.
That raises the obvious question. We do not use the Name that way now. Tosafot answer that the danger belongs to the Temple, but outside it looking is still forbidden for another reason: it pulls your mind away. They quote a rabbi in the Talmud of the Land of Israel who says, in effect, that the only issue is distraction, and that he looks without losing focus.
A Mishnah in Megillah shows the same worry. Rabbi Yehuda says that even a kohen whose hands are stained with dye should not bless, because the people look at him.
So the sources give two layers. One is a warning about eyesight, tied to the Temple. The other is about attention. The Mishnah Berurah (a central commentary on daily Jewish law) says that today the law only forbids a long stare, which pulls your mind away, and a quick look is allowed. Still, he says, the custom now is not to look at them at all, as a reminder of the Temple.
Shulchan Aruch, Orach Chaim 128:23, with the Rema,Chagigah 16a,Tosafot, Chagigah 16a,Mishnah Megillah 4:7,Mishnah Berurah 128:89