Good question. There is a mitzvah in the Torah for a kohen to bless the Jewish people every day with upraised hands. This is called Birkat Kohanim (“Blessing of the Priests”) or Nesiat Kapaim (“Raising the Palms”). This mitzvah is not restricted to Temple times nor is it limited to the Land of Israel.1 So nowadays why is this blessing commonly practiced on holidays, only?

To be sure, many communities in Israel, as well as many Sefardic communities in the Diaspora, do Birkat Kohanim every day (or every Shabbat). But why don’t Ashkenazic communities outside of Israel do so as well?

Only in a Time of Joy

Rabbi Schneur Zalman of Liadi explains in Shulchan Aruch Harav:

The custom in all these lands [e.g., Ashkenazic custom] is that the Priestly Blessing is conferred only on yom tov [i.e., holiday], because then people are in a joyful and festive mood—and “only one who is glad of heart shall bless.” On other days, by contrast, even on Shabbat, [the kohanim] are preoccupied with their livelihood and their loss of working hours, and they are not in a happy state of mind. Indeed, even on yom tov the Priestly Blessing is conferred only in the Mussaf service,when [directly afterwards] they will leave the synagogue and partake of refreshments and rejoice in the celebration of the festival.2

In other words, since the Priestly Blessing is meant to be conferred in a spirit of joy, and the holidays specifically are a time to rejoice, it is performed only then.

Immersion

Rabbi Schneur Zalman presents another explanation3 as to why the blessing is only bestowed on yom tov. Many Kohanim make sure to immerse in the mikvah before Birkat Kohanim. Although it is common for any man to enter the mikvah before a holiday, the kohanim would immerse on the eve of yom tov and avoid engaging in marital relations that night. In doing so, they would be pure the following day when giving the blessing.

On Shabbat, however, they didn’t want to forfeit cohabitating with their wives but (for various reasons) would not go to the mikvah on the day of shabbat.4 Thus, they refrained from giving the Priestly Blessing on Shabbat even though they had immersed the day before.

This explains the custom in some communities (not Chabad), where the kohanim refrain from Birkat Kohanim when Shabbat coincides with a holiday. The joy was there, but it was likely that the kohanim become impure the night before and had not immersed in the interim.5

Merely Justification

After listing the above reasons, Rabbi Schneur Zalman concludes that the real reason behind this custom remains elusive:

…All of the above reasons are given merely to justify the above-described custom [of refraining from conferring the Priestly Blessing]. Though these reasons are not of sufficient weight to override a positive Scriptural mitzvah, [people who follow this custom] have not transgressed any [mitzvah] as long as the announcement “Kohanim!” is not made [calling them to bless the people]. Nevertheless, encouragement is due to the inhabitants of Eretz Yisrael and the surrounding lands, who confer the Priestly Blessing every day as the Sages ordained, thereby fulfilling three positive mitzvot every day, and who are not concerned about [the need for] immersion.

Indeed, it is for this very reason that many, including Rabbi Yosef Caro6 (whose words are partially quoted by the Shulchan Aruch Harav), held that even in the Diaspora, kohanim should confer Birkat Kohanim every day. This is indeed the custom of many Sefardim who generally follow Rabbi Yosef Caro’s rulings.

Mysterious Obstacles From Heaven

The Lubavitcher Rebbe—Rabbi Menachem M. Schneerson, of righteous memory—relates that Rabbi Schneur Zalman greatly desired to institute the daily performance of the Priestly Blessing in the Diaspora but for an unknown reason held back from doing so.7

A strikingly a similar tradition is found regarding contemporary, but decidedly non-Chassidic rabbis as well. Rabbi Chaim Volozhin was the main disciple of Rabbi Eliyahu of Vilna, known as the Vilna Gaon. He notes that his teacher greatly desired that Birkat Kohanim be instituted every day in his synagogue. After hesitating, he finally decided that the next day the kohanim of his synagogue would do Birkat Kohanim. That very same day he was placed under arrest by government authorities. Apparently, Heaven prevented him from acting on the matter.

Rabbi Chaim continued that many years later, he too instructed in his own synagogue that on the next day the Birkat Kohanim should be recited. But that night the synagogue burned down. From all this it was clear to him that the matter was not supported by Heaven.8

Blessings Everywhere

The Talmud tells us that the Priestly Blessing extends even to the workers in the surrounding fields who are unable to attend.9 In the same vein, says the Rebbe, the daily blessings given in the Land of Israel are diffused throughout the Diaspora.10

Nonetheless, we all pray for the era of Moshiach, when the Priestly Blessing will finally be recited in the Holy Temple,11 may it be speedily in our days!