1) BIRKAS KOHANIM: WHOSE MITZVAH?
OPINIONS: The Mishnah and Gemara discuss the Mitzvah of Birkas Kohanim. The Torah commands the Kohanim to bless the people. Is the Torah's Mitzvah directed only to the Kohanim (it is their Mitzvah to bless) or also to those who are being blessed (they have a Mitzvah to be blessed)?
The BI'UR HALACHAH (beginning of Hilchos Nesi'as Kapayim, OC 128) cites the SEFER CHAREDIM who writes that the Yisraelim who stand facing the Kohanim quietly with intention to receive the blessing as Hash-m commanded are included in the Mitzvah. The SEFER HAFLA'AH in Kesuvos (25a), MESHECH CHOCHMAH (Parshas Naso), MINCHAS CHINUCH (#378) and other Acharonim understand that just as the Kohanim have a Mitzvah to give the blessing, so, too, there the Yisraelim have a Mitzvah to receive it. What is the source for the Mitzvah for Yisraelim to receive the blessing?
RAV BARUCH WEISS in SEFER BIRKAS HORAI (1:9) suggests two possible sources. TOSFOS (DH u'Kesiv) writes that every Korban Tzibur (such as the Tamid and Musaf) must be accompanied by Birkas Kohanim. Tosfos apparently understands that Birkas Kohanim is part of the service of a Korban Tzibur. Since the Tzibur is obligated to ensure that the Korban is offered even though it is the Kohanim who actually perform the service, the Tzibur is also obligated to ensure that the Kohanim perform Birkas Kohanim when the Korban is offered. Since the daily Tefilos were instituted to correspond to the daily Korbenos Tamid (Berachos 25b), perhaps the Tzibur is obligated to ensure that the Kohanim perform Birkas Kohanim at every Tefilah. (The Rabanan disestablished the practice of reciting Birkas Kohanim at Minchah. See Ta'anis 26a.)
Another possible source is the statement of Abaye, who derives from the verse, "Emor Lahem" (Bamidbar 6:23), that the Tzibur is supposed to request from the Kohanim to go up and bless them. Abaye's teaching may mean that it is an obligation of the Yisraelim to have the Kohanim bless them. (See also Birkas Horai 8:17.)
(b) However, the RITVA in Sukah (31a) writes that the Yisraelim have no Mitzvah to have the Kohanim bless them. The Mitzvah is only the Kohanim's obligation to bless the Yisraelim. (The Ritva discusses why the prohibition of Bal Tosif does not prohibit a Kohen from reciting Birkas Kohanim for a second Minyan after he has already recited it for an earlier Minyan.) The MAGEN GIBORIM (OC 128) writes that the Ritva obviously disagrees with the Sefer Charedim, and therefore the Halachah does not follow the view of the Sefer Charedim. (See DEVAR AVRAHAM 1:31, who attempts to reconcile the two views.)
(c) The BIRKAS HORAI favors the opinion of the MAHARI ASAD (#46) who writes that the Sefer Charedim does not mean that the Yisraelim are obligated to receive a blessing. Rather, he means simply that when the Yisrael does what is necessary for the Kohanim to bless him (he faces towards the Kohanim and listens to the blessing), he helps the Kohanim perform their Mitzvah, and therefore he receives reward for the Mitzvah which he helped the Kohanim perform. (Although the Kohanim may recite Birkas Kohanim when no Yisrael is present, nevertheless a Yisrael who is present is supposed to face the Kohanim and listen to the blessing.) Accordingly, the Mitzvah of Birkas Kohanim is only an obligation of the Kohanim.
2) HALACHAH: BIRKAS KOHANIM EVERY DAY
OPINIONS: The Gemara teaches that a Kohen who does not go up to perform Birkas Kohanim transgresses three Mitzvos Aseh. Even if he goes up some of the time, such as on Yom Tov, but he does not go up every day, he transgresses these three Mitzvos Aseh.
How can the practice of those Kohanim who do not go up to perform Birkas Kohanim every day be justified?
(a) The ROSH in Megilah (3:22) rules, based on the Yerushalmi, that a Kohen transgresses the Isur Aseh only when he hears the Kohanim being called to perform Birkas Kohanim and he does not respond. If he walks out before the Kohanim are called up, he does not transgress a Mitzvas Aseh. The TUR (beginning of OC 128) cites support for this explanation from the Targum on the verse, "Emor Lahem..." (Bamidbar 6:23), who translates the words as, "When you tell them...," implying that the Kohanim are commanded to perform Birkas Kohanim only when they are directed to do so (by being called up).
Although a Kohen who avoids being called up does not transgress an Aseh, nevertheless he does not fulfill the three Mitzvos Aseh of Birkas Kohanim. Therefore, this explanation still provides no valid reason for a Kohen to step outside l'Chatchilah when the Kohanim are called up in order to avoid having to perform Birkas Kohanim.
(b) The SHIBOLEI HA'LEKET, cited by the BEIS YOSEF, quotes the ruling of RABEINU YITZCHAK BEN YEHUDAH (a mentor of Rashi) who explains that a mourner (Avel) who is a Kohen should not be permitted to recite Birkas Kohanim, because Birkas Kohanim requires Simchah and an Avel has no Simchah. (This ruling applies to an Avel only when his Aveilus still has Halachic significance -- "as long as he still sits in a different place due to the Avelus," which is thirty days for a relative and twelve months for a parent.) The REMA (OC 128:44) and the VILNA GA'ON write that his source is the Gemara here which quotes the verse, "Tov (Lev) [Ayin] Hu Yevorach" (Mishlei 22:9), and says that it should be read "Hu Yevarech" -- "he shall bless," meaning that he shall bless when he is happy, "Tov Ayin."
Although the Rema accepts this ruling (OC 128:43), the BEIS YOSEF argues that there is no source in the Gemara for this ruling, and therefore the exemption from Birkas Kohanim should be limited to the seven days of Aveilus.
In practice, a Kohen who is an Avel should leave the synagogue before the Kohanim are called up to perform Birkas Kohanim, as mentioned above.
(c) RABEINU YITZCHAK BEN YEHUDAH (ibid.) extends the exemption from Birkas Kohanim to include an unmarried man ("Bachur"), based on the Gemara in Yevamos (62b) which says that a person who has no wife dwells with no joy. The RASHBA (Teshuvos 1:85) writes that although he did not find a definitive source for this ruling, it may be based on a Midrash. Indeed, the Zohar cited by the MAGEN AVRAHAM (128:64) writes that the Shechinah does not rest upon a Bachur, and therefore a Bachur who is a Kohen should not perform the Avodah ("the Shechinah rests only on a person through the joy of a Mitzvah"; Pesachim 117a).
The Poskim reject the ruling of Rabeinu Yitzchak ben Yehudah based on the Gemara which says that a Katan may perform Birkas Kohanim (under certain circumstances) even though he is not married. Nevertheless, the Rema (OC 128:44) writes that if a Bachur does not want to perform Birkas Kohanim, he cannot be forced to do it, as long as he leaves the synagogue before the Kohanim are called up.
(d) The MAHARIL (Chadashos #21), cited by the BEIS YOSEF, was asked why in many parts of Europe ("Ashkenaz") the Kohanim performed Birkas Kohanim only on Yom Tov. He suggested two sources for the practice. According to some authorities, a Kohen must immerse in a Mikvah prior to performing Birkas Kohanim. Since it was too difficult to immerse every day in most parts of Europe, the practice to recite Birkas Kohanim every day was discontinued.
He suggests further that performing Birkas Kohanim every day would cause "Bitul Melachah" and create a burden on workers who need to be at work on time.
The BEIS YOSEF strongly rejects these reasons. Since Tevilah is not mentioned in the Gemara, the inability to fulfill the stringency of immersing in a Mikvah prior to performing Birkas Kohanim is no reason to annul the Mitzvas Aseh d'Oraisa of Birkas Kohanim. The reason of Bitul Melachah is difficult to understand, because Birkas Kohanim does not take so much time that it should warrant this exemption. The Beis Yosef writes that, indeed, in all of Eretz Yisrael and Egypt the practice is for the Kohanim to perform Birkas Kohanim every day.
The DARCHEI MOSHE, however, defends the second reason of the Maharil. Bitul Melachah does not mean arriving at work late, but rather it means that in Chutz la'Aretz people toil so arduously to earn a living that they are unable to arouse the degree of Simchah necessary to perform Birkas Kohanim. Even on Shabbos, their minds are so distracted by their past suffering and future worries about earning a living that they do not reach the proper level of Simchah. The only time they are able to perform Birkas Kohanim with the appropriate degree of Simchah is on Yom Tov, when the Mitzvah of "v'Samachta b'Chagecha" makes them truly joyful and enable them to perform Birkas Kohanim with Simchah.
In the Shulchan Aruch, the Rema adds that even on Yom Tov, the Kohanim perform Birkas Kohanim only during Tefilas Musaf when they are about to leave the synagogue and experience the joy of Simchas Yom Tov. This is the source for the practice of most communities outside of Eretz Yisrael.
The Vilna Ga'on is rumored to have attempted to re-institute Birkas Kohanim on a daily basis in his community, but he received a sign from heaven not to do so. When his students arrived in Eretz Yisrael, they revived the practice of the daily Birkas Kohanim among the Ashkenazic communities. Nowadays, many Ashkenazic communities in Eretz Yisrael perform Birkas Kohanim every day, although some communities (such as those of Haifa and most communities in the Galilee) perform Birkas Kohanim only on Shabbos and Yom Tov. In Chutz la'Aretz, only Sefardic communities perform Birkas Kohanim on a daily basis.