HOW OFTEN MUST KOHANIM CUT THEIR HAIR?
(Beraisa): Kohanim of the Mishmar (that is serving this week) may drink wine at night, but not during the day, lest the Beis Av (the division of the Mishmar serving today) will need help. Kohanim of today's Beis Av may not drink wine at night or during the day, for they are totally engaged in Avodah;
Therefore, a Kohen who knows his (ancestors') Mishmar and Beis Av and the day it serves may not drink wine that day. If he knows his Mishmar but not his Beis Av, he may not drink the entire week. If he does not know his Mishmar and Beis Av, but knows that the Mishmar served (e.g. it was not disqualified due to Chalalim), he may not drink the entire year.
Rebbi says, I say that he is (i.e. should be) always forbidden to drink wine. However, his Kilkul (dilemma) is his solution.
(Abaye): Nowadays, Kohanim drink wine. This is like Rebbi.
(Beraisa): A Kohen Hedyot cuts his hair once every 30 days.
We learn from a Gezeirah Shavah. It says about Kohanim "...U'Fera Lo Yishalechu", like it says about a Nazir "Gadel Pera Se'ar Rosho". Just like there it means 30 days of growth, also here.
Question (Rav Papa): Perhaps a Kohen may not grow his hair at all!
Answer (Abaye): Had it said 'Lo Yishalechu Pera', that would be correct. However, it says "U'Fera Lo Yishalechu". They may have Pera (30 days growth), but no more.
Question: If so (we learn from a verse,) this should apply even nowadays!
Answer: The Isur of long hair is like drinking wine. It applies only in the Mikdash.
Question (Beraisa - Rebbi): I say that a Kohen is always forbidden to drink wine. However, his Kilkul is his solution.
(Abaye): Nowadays, Kohanim follow Rebbi, who permits them to drink wine.
Inference: Chachamim forbid.
Summation of question: Chachamim forbid so that Kohanim will be ready to serve if the Mikdash is suddenly rebuilt. For the same reason they should need to cut their hair every 30 days!
Answer #1: If the Mikdash is suddenly rebuilt, a long-haired Kohen could cut his hair and serve.
Question: Likewise, a drunk Kohen could nap and serve (yet Chachamim forbid them to drink)!
(Rav Acha) Walking a 2000 Amos or sleeping any amount negates the effects of wine.
Answer: Rav Nachman taught that that is for one who drank only one Revi'is. If he drank more, walking or sleep will worsen his condition!
Answer #2 (Rav Ashi): Shtuyei Yayin (serving when drunk) is Mechalel (disqualifies) the Avodah, therefore Chachamim decreed about it. Pru'ei Rosh (serving with long hair) does not Mechalel Avodah, therefore Chachamim did not decree about it.
Question (Beraisa): The following are Chayav Misah: Pru'ei Rosh and Shtuyei Yayin.
Question: We know the source for Shtuyei Yayin - "Yayin v'Shechar Al Tesht...v'Lo Samusu". What is the source for Pru'ei Rosh?
Answer: The Torah equates this to Shtuyei Yayin (they are written adjacently). Just like Shtuyei Yayin is Chayav Misah, also Pru'ei Rosh.
Summation of question: Likewise, we should equate them and say that Pru'ei Rosh is also Mechalel Avodah!
This is left difficult.
Question (Ravina): How did we know the law before Yechezkeil said this verse?
Counter-question (Rav Ashi): "Kol Ben Nechar Erel Lev v'Erel Basar Lo Yavo El Mikdashi" (a Rasha or Arel may not enter the Mikdash). How did we know the law before Yechezkeil?
Answer to both questions (Rav Ashi): You must say that it was a tradition from Sinai. Yechezkeil wrote a verse alluding to it.
Rambam (Hilchos Bi'as Makdish 1:7): If a Kohen knows his Mishmar and Beis Av and that today is the day his Beis Av serves, he may not drink wine today. If he knows his Mishmar but not his Beis Av, he may not drink wine the entire week that his Mishmar serves. If he does not know his Mishmar nor his Beis Av, he should be forbidden to drink always. However, his Kilkul is his solution. He may drink at any time, for he cannot serve until his Beis Av and Mishmar are determined.
Rebuttal (Ra'avad): When the third Mikdash is built, there will be a Navi to tell everyone his Beis Av and Mishmar and which day he serves. There will be no delay! Rather, the Halachah follows Rebbi. Now there is no Mikdash, and we are not concerned lest it be built immediately. Therefore, whether or not a Kohen knows his Beis Av and Mishmar he may drink at any time. Abaye said that nowadays Kohanim drink according to Rebbi. He did not distinguish (whether or not he knows his Beis Av and Mishmar).
Defense #1 (Ri Korkus and Kesef Mishneh): The Ra'avad explains Rebbi like Rashi, that all Kohanim should be forbidden, lest Mishmaros change when the Mikdash will be built, or lest Chanukas ha'Bayis will require all the Kohanim. However, Rebbi permits due to Kilkul, the lack of a Mikdash. He is not concerned lest it be built. This is difficult, for throughout Shas we anticipate that it be rebuilt speedily! One who says 'I am a Nazir on the day the Mikdash is rebuilt' may not drink wine (Eruvin 43a)! The Rambam explains that also Rebbi forbids one who knows his Beis Av and Mishmar. He argues only about one who does not know. Even if the Mikdash will be built b'Yedei Shomayim, they will not inform Kohanim about their Mishmaros the same day. Also, the Rambam (Hilchos Beis ha'Bechirah 6:15) permits Hakravah without a Mikdash. Perhaps the Rambam does not teach that even after it is built a Kohen may drink before he finds out which is his Mishmar. Rather, he teaches why a Kohen is permitted nowadays, i.e. due to Kilkul, i.e. not knowing his Mishmar.
Question: According to the Rambam, why did Rebbi say 'I say that Kohanim are always forbidden to drink'? Rebbi discusses only one who does not know his Mishmar. Chachamim truly forbid him always!
Answer (Ri Korkus): Indeed, Rebbi means 'I agree that Kohanim should be always forbidden.' Perhaps he said this to clarify that he discusses only the last clause, about one who does not know his Mishmar.
Defense #2 (Me'iri): The Rambam discusses when the Mikdash stands, like Chachamim in the Beraisa. He explains, like Rashi and the Ra'avad, that Rebbi teaches that after the Churban we should forbid all Kohanim, but Kilkul, the lack of a Mikdash, permits.
Question (Ran Sanhedrin 22b DH Michlal): Why did the Gemara need to infer from Rebbi that Chachamim forbid nowadays? The Gemara is Medayek from the Reisha what is explicit in the Seifa, but it is not Medayek from the Seifa what is explicit in the Reisha!
Answer (Ran): We know that Chachamim discuss nowadays only through Rebbi.
R. Chananel (17a-b): Rebbi permits Kohanim who do not know their Mishmar to drink wine. Abaye knew that he was from Beis Eli. This is why he did not drink wine (Kesuvos 65a).
Note: The Gemara asked a question by proving that Chachamim forbid even nowadays. Seemingly, just like Kohanim rely on Rebbi to drink, they rely on him to grow long hair! According to R. Chananel (and the Rambam), Rebbi permits only confused Kohanim. We ask why even Kohanim who know their Mishmar may grow long hair. According to Rashi and the Ra'avad, the question is from a Diyuk. Abaye said that Kohanim rely on Rebbi (only) regarding wine. They do not rely on him regarding long hair! (Perhaps we think that this is because it is easier to be stringent about hair.)
Rambam (10): Just like Kohanim are commanded not to drink only b'Sha'as Bi'as Mikdash, the same applies to Gadel Pera (growing long hair).
Rebuttal #1 (Ra'avad): Bi'as Mikdash is when they enter with their Mishmar. Kohanim are always commanded - "U'Fera Lo Yishalechu"!
Defense of #1 (Migdal Oz, brought in Kesef Mishneh): The Rambam exempts only when there is no Mikdash. He agrees that when the Mikdash stands, Kohanim are commanded the entire year.
Defense #2 (Ri Korkus): Even when the Mikdash stands, Kohanim are commanded only at the time they serve. "U'Fera Lo Yishalechu" refers to a Kohen Gadol. We learn from the Seifa of the verse, "Kasom Yichsemu Es Rosheihem", which refers to the Kohen Gadol's haircut (Sanhedrin 22b). The same applies to a Kohen Hedyot when he serves. The stringency of a Kohen Gadol over a Hedyot is only that he must be ready to serve at all times.
Rebuttal #2 (Ramban, Hasagos on Sefer ha'Mitzvos Lavin 73 and 153): The Torah forbids Shtuyei Yayin and Pru'ei Rosh only at the time of Avodah. The Isur to enter the Mikdash thusly (without doing Avodah) is only mid'Rabanan.
Ran (ibid., DH v'Yalif): A Gezeirah Shavah is only between verses of Chumash. The 'Gezeirah Shavah' to the verse in Yechezkeil is really a Giluy Milsa (merely reveals what is Pera).
Rambam (11): Thirty days of growth is called Pera, like it says about a Nazir. Therefore, a Kohen Hedyot who serves must shave every 30 days.
Sefer ha'Chinuch (152 DH mi'Dinei): Thirty days of growth is not Nivul (repulsive). A Yisrael may enter the Mikdash as long as his hair is not so long that it is Nivul. The Torah is more stringent about Kohanim.
Matzpas Eisan (Sanhedrin 22b): Why are Kohanim allowed to enter Shabbos or Yom Tov with long hair? Rashi and Tosfos (Sukah 41a DH Ela d'Ivni and DH I Nami) say that we are concerned lest the Mikdash be built on Shabbos and Yom Tov. All agree that Melachah (e.g. cutting hair) to Machshir a Kohen for Avodah is Tzerichah l'Gufah. Only R. Eliezer holds that Machshirim are Docheh Shabbos. Chachamim must say that Nochrim would shave Kohanim.
Note: Eruvin 43b concludes that Moshi'ach will not come on Shabbos. Tosfos (43b DH v'Asur) equates this with concern for Binyan ha'Mikdash.
Teshuvas Toras Lishmah (388 DH Teshuvah): Even though thirty days growth is not Nivul, Al Pi Sod men should shave before Yom Kipur to remove the forces of Din. One who has not shaved in 30 days should not be Shali'ach Tzibur for Yom Kipur, in particular to say the Avodah.