Surprisingly, the custom among Ashkenazic communities outside Israel is to recite Birkat Kohanim (the priestly blessing) only during Musaf on Yom Tob. Although the Halacha seems very straightforward, that the Kohanim are to bless the congregation every day, nevertheless, the custom among Ashkenazim is for this Beracha to be pronounced only during Musaf on Yom Tob.
This custom is mentioned already by the Rama (Rav Moshe Isserles of Cracow, 1530-1572), in his glosses to the Shulhan Aruch (Orah Haim 128). He suggests that the reason might be that this blessing must be recited with love and joy, as the Kohanim say in the introductory Beracha, "Asher Kideshanu…Ve’sivanu Le’barech Et Amo Yisrael Be’ahaba" – "…who has sanctified us…and commanded to bless His nation, Israel, with love." (Rav Eliezer Papo, in his work Elef Ha’magen, suggests that the source of this concept is G-d’s instruction to Moshe, "Amor Lahem" – to tell the Kohanim about the Misva of Birkat Kohanim. As we know, several expressions in the Torah are shared by other languages, and Rav Papo thus proposed that the word "Amor" here refers to the French word "Amor," which means "love," indicating that the Kohanim must administer this blessing with great joy and affection.) The hardships faced by the Jews of Eastern Europe, the Rama writes, made it very difficult for them to experience the level of Simha (joy) required for the recitation of Birkat Kohanim. It is perhaps for this reason that the custom developed to recite this Beracha only during Musaf on Yom Tob, when the congregation was about to return home to enjoy a festive meal in celebration of the holiday.
The Taz (Rav David Ha’levi Segal, 1586-1667) offered a different reason, suggesting that the custom was to require the Kohanim to immerse in a Mikveh before pronouncing this blessing. As it was very difficult to immerse in a Mikveh every day, the custom developed to recite Birkat Kohanim only on Yom Tob.
The Magen Abraham (Rav Abraham Gombiner, 1633-1683) cites Rav Menahem Azarya de Fano (Italy, 1548-1620) as remarking about this Ashkenazic practice, "Hu Minhag Garu’a" – "It is an improper practice." Birkat Kohanim is a Biblical Misva that has great value, and thus it should be performed each day. Indeed, the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) writes that he has no explanation for why Ashkenazic communities do not perform Birkat Kohanim each day, but it seems that it was decreed by G-d that this should not be done. The Aruch Ha’shulhan alludes to two instances of great Ashkenazic sages who tried to reinstate the daily recitation of Birkat Kohanim, but calamity struck just before they managed to do so. The Vilna Gaon (Rav Eliyahu of Vilna, 1720-1797) decided he would reinstate the daily recitation of Birkat Kohanim, but the day before the Beracha was to be recited, he was imprisoned. His disciple, Rav Chaim of Volozhin (1749-1821), also tried, but the night before he intended on having the Kohanim recite the Beracha, half the city – including the synagogue – burned down. Ashkenazic Rabbis viewed these extraordinary events as an indication that G-d did not wish for them to reinstate the daily recitation of Birkat Kohanim.
Our community, of course, follows the practice of reciting Birkat Kohanim each day. This is a great privilege, as we have the opportunity to receive the great blessings which the Kohanim bring down from the heavens through this Beracha, each and every day. The Keli Yakar (Rav Shlomo Efrayim Luntschitz, 1550-1619), commenting to Parashat Naso, writes that the Hazzan, who dictates the words of Birkat Kohanim to the Kohanim, serves as the "pipeline" which brings the blessing from the heavens. He transfers this blessing to the Kohanim, who then, through the recitation of the Beracha, shower this blessing upon the people. Some commentators explain on this basis why Birkat Kohanim is referred to in our prayers as "Beracha Ha’meshuleshet" ("the triple blessing"). The simple meaning is that the Beracha consists of three statements, but some explain that this expression refers to the three parties involved in the process of Birkat Kohanim – the Hazzan, the Kohanim, and the people.
Rav Moshe Sternbuch, a contemporary Ashkenazic Sage in Jerusalem, had the practice that whenever he needed to travel abroad, he would pay two Kohanim in Israel to have him in mind when they recited Birkat Kohanim each day while he was away. Since he would not have the opportunity to hear Birkat Kohanim while visiting Ashkenazic communities abroad, he ensured not to forfeit the great benefits of this Beracha by having two Kohanim include him in their blessings during his absence.
Summary: Ashkenazic communities in the Diaspora have the practice to recite Birkat Kohanim only during Musaf on Yom Tob, despite the fact that the Misva seems to require its recitation each day. Indeed, a number of Ashkenazic Poskim struggled to explain this custom, but it is told that on two occasions, when great Ashkenazic Rabbis wanted to reinstate the daily recitation of Birkat Kohanim, tragedies befell the community – signaling G-d’s disapproval of the effort to change this custom. In any event, Sephardic custom is to recite this Beracha each day even in the Diaspora, and we should feel privileged and fortunate to be able to receive this special Beracha each day, which brings great blessing from the heavens.