Why do kohanim in Ashkenazi synagogues outside of Eretz Yisrael only bless the congregation on Yom Tov, and what should an Ashkenazi kohen do when praying in a Sepharadi minyan during any other day?
The Rama says (siman 128 se’if 44) in the name of Rabbi Meir of Rottenburg, the Sefer Hasidim, and others, that their custom was to only do Birkat Kohanim on Yom Tov because there is a special joy on those days, whereas during other times people are preoccupied with their livelihood. However, this is difficult to understand because we are discussing a Biblical mitzvah so how can it be set aside just because people aren’t happy enough?
The responsa Bet Efrayim (Rav Ephraim Zalman Margoliot) gave an additional reason that today’s Kohanim are possibly not true Kohanim, so how can we force them to bless if when a non-Kohen blesses he transgresses a sin. He gives another reason, in the name of Hagahot Maimoniyot, that there is a custom for Kohanim to immerse in a mikveh before blessing, and since today this is not done it can be an additional reason why they do not bless daily. And the Bet Efrayim ends by saying that anyone who tries to change this custom of only blessing on Yom Tov is denigrating the honor of the Rishonim, as if they disregarded the mitzvah of Birkat Kohanim.
On the other hand, some say that the Ashkenazi custom not to bless during the year should be changed. The book Knesset HaGedolah (in Hagahot Bet Yosef) says in the name of the Rema miPano (Rav Menachem Azariah from Pano) that this custom where Kohanim do not bless during the whole year except for Yom Tov – neither during the week nor on Shabbat – is a bad custom. Also Rav Hesed LeAvraham says (item 1, mentioned by Kaf HaHaim item 16) that much good is lost when the Kohanim do not bless. And it is mentioned in the book Mayim Haim (siman 4) that based on the above opinions, this custom should be changed and they should bless every day.
It seems that according to halacha our bretheren Ashkenazim should continue with their existing custom, since that was the opinion of their great Rabbis for many generations from the time of the Rishonim – more than 700 years. This was the opinion of Rabbi Meir of Rottenburg, the Maharil (Rav Yaakov Moelin), Darchei Moshe, the Taz, and the Magen Avraham. G-d forbid, one should not defame the Rishonim, as stated by the Bet Efrayim. And the Bet Yosef did not write anything against this custom per se, but he did say more power (Yasher Koah) to those in Eretz Yisrael and Egypt who do Birkat Kohanim every day.
However, if an Ashkenazi Kohen finds himself in a Sepharadi synagogue, he should go up together with his Sepharadi bretheren to bless the congregation because one is not allowed to deviate from the custom of the place in which he finds himself. And since many do say that Ashkenazim should really change their custom, that is one more reason for him to bless along with the Sepharadi Kohanim. This is indeed the opinion of Rav Shlomo Zalman Auerbach in his book Shulhan Shelomo, as well as that of Rav Moshe Feinstein in his responsa Rivevot Efrayim (part 6 siman 57).
It is also worth noting that many great Ashkenazi Rabbis who live outside of Eretz Yisrael, despite the fact that it is not their custom to be blessed by Kohanim daily, search for a Sepharadi synagogue in order to hear the Birkat Kohanim. This was the custom of Rav Yehezkel Serna (as mentioned in responsa HaReah HaTov, page 330), and Rav Shalom Shvadron (as mentioned in the book Kol Hotzev), as well as Rav Aryeh Leib Shteinman.
Likewise, in our own community in New Jersey, Rav Selsberger shlit”a comes to us in Congregation Etz Ahaim to hear Birkat Kohanim.