According to this Gemara, is there any way to justify the practice of those
Kohanim who do not go up to perform Birkas Kohanim every day?
(a) The ROSH in Megilah (3:22) cites the Yerushalmi and rules that a Kohen
only transgresses the Isur Aseh if 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, then he does not transgress a Mitzvas Aseh. The TUR
(beginning of OC 128) cites support for this 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 do Birkas Kohanim
only when they are told 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, there is still no reason for a Kohen to step outside 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 says that an Avel who
is a Kohen -- as long as his Avelus still has Halachic significance ("as
long as he is still sitting in a different place due to the Avelus," or
thirty days for a relative and twelve months for a parent) -- should not be
able to recite Birkas Kohanim, because Birkas Kohanim requires Simchah, and
an Avel does not have Simchah. The REMA (OC 128:44) and the VILNA GA'ON
write that his source is our Gemara 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.
Although the Rema accepts this ruling (OC 128:43), the BEIS YOSEF argues
that there is no source in the Gemara for this, and therefore we should
limit the exemption from Birkas Kohanim to the seven days of Avelus.
In any case, the Kohen who is an Avel should leave the synagogue before he
is called to 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) that says that a person who has no wife, has no joy. The
RASHBA (Teshuvos 1:85) writes that although he never found a definitive
source for this ruling, it might 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 only rests on a person through the joy of a Mitzvah;"
Pesachim 117a).
The Poskim reject the ruling of Rabeinu Yitzchak ben Yehudah based on the
Gemaras which say 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 do Birkas Kohanim, then we
cannot force him to, 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 it
is that in much of Europe ("Ashkenaz") the Kohanim only performed Birkas
Kohanim on Yamim Tovim. He suggested two sources for the practice. First, it
is because some say that a Kohen must immerse in a Mikvah before performing
Birkas Kohanim, and it was too difficult to immerse every day in most of
Europe.
Second, he says that doing Birkas Kohanim every day would cause "Bitul
Melachah" and create a burden on workers who needed to be at work on time.
The BEIS YOSEF strongly rejects these reasons. Since Tevilah is not
mentioned in the Gemara, why should that stringency -- the Chumra of
immersing in a Mikvah before performing Birkas Kohanim -- cause one to annul
the Mitzvas Aseh d'Oraisa of giving the Berachah. Moreover, 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 Israel and Egypt the Kohanim performed Birkas
Kohanim every day.
The DARKEI MOSHE, however, defends the second reason of the Maharil,
explaining that Bitul Melachah does not mean a delay from working, but
rather it means that in Chutz la'Aretz, people toil so hard to earn a living
that they are not able to arouse the proper Simchah that is necessary in
order to perform Birkas Kohanim. Even on Shabbos, their minds are so
distracted by their past and future plans for earning a living that they do
not reach the proper level of Simchah. The only time they are able to
perform the Birkas Kohanim with the appropriate degree of Simchah is on Yom
Tov, when the Mitzvah of "v'Samachta b'Chagecha" makes them truly joyful,
enabling 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 Israel.
The Vilna Ga'on is said 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 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.