The Kohanim bless the people at four times (Shacharis, Musaf, Minchah and Ne'ilah) on three occasions - fasts, Ma'amados, and Yom Kippur.

The function of Ma'amados is as follows. The Torah tells everyone to bring Korbanos, but they cannot be brought in the absence of their owners.

The Nevi'im therefore decreed that there be twenty-four Mishmaros of Kohanim, paralleled by twenty-four Ma'amados of Kohanim, Levi'im and Yisraelim.

When each Mishmar had its turn, the Kohanim and Levi'im would go to serve in the Beis ha'Mikdash and the Yisraelim would gather and read from Ma'ase Bereishis according to each day of the week.

The larger sections would be read by two people, the smaller by one.

At Shacharis, Musaf and Minchah, everyone would recite it for themselves.

On Erev Shabbos at Minchah it was not done, out of respect for Shabbos.

(R. Akiva): On any day where there was Hallel, there was no Ma'amad for Shacharis.

On any day where there was a Korban Musaf, there was no Ma'amad for Ne'ilah.

On any day where there was a Korban Eitzim, there was no Ma'amad for Minchah.

(Ben Azai): R. Yehoshua learned as follows:

On any day where there was a Korban Musaf, there was no Ma'amad for Minchah.

On any day where there was a Korban Eitzim, there was no Ma'amad for Ne'ilah.

R. Akiva retracted and followed Ben Azai.

There were nine occasions when people would donate wood for the pyres and bring Korbanos (as listed).

On the first of Teves there was no Ma'amad as there was Hallel, Korban Musaf, and a Korban Eitzim.

Five calamities befell our ancestors on the Seventeenth of Tamuz:

The Luchos were broken;

The bringing of the Tamid was annulled;

The walls of Jerusalem were breached;

Apostemus burned a Sefer Torah;

An idol was placed in the Beis ha'Mikdash.

...and five on Tish'ah b'Av:

It was decreed that our ancestors would not enter Eretz Yisrael;

The first Beis ha'Mikdash was destroyed;

The second Beis ha'Mikdash was destroyed;

Beitar was captured;

Jerusalem was razed to the ground.

From the beginning of Av, pleasure is curtailed.

During the week in which Tish'ah b'Av falls, it is forbidden to have haircuts or wash clothing, except on Thursday out of respect for Shabbos.

On Erev Tish'ah b'Av, a person may not eat two cooked foods, eat meat or drink wine.

(R. Shimon b. Gamliel): He should change his custom.

(R. Yehudah): He should overturn his bed,

(Chachamim): He need not overturn his bed.

(R. Shimon b. Gamliel): There were never days as festive as the fifteenth of Av and Yom Kippur:

The girls would dress in white garments that were borrowed, so as not to embarrass those who lacked any.

All the garments required Tevilah first.

They would dance in the vineyards, and say that the men should look at their families rather than their beauty.

The Pasuk speaks of Shlomo being crowned on his wedding day, which refers to the giving of the Torah;

And it speaks of the day of his heart's rejoicing, which refers to the rebuilding of the Beis ha'Mikdash.

Question: The Mishnah said that the Kohanim bless the people at four times (Shacharis, Musaf, Minchah and Ne'ilah) on three occasions - fasts, Ma'amados, and Yom Kipur; but there is no Musaf on fasts and Ma'amados!

Answer: It means that on three occasions, the Kohanim would bless the people during every prayer, which sometimes resulted in four times.

(R. Nachman citing Rabah bar Avuha): Such is R. Meir's opinion, but the Chachamim hold that Kohanim never bless during Minchah or Ne'ilah.

The Chachamim mentioned refers to R. Yehudah:

(R. Meir): the Kohanim bless during Shacharis, Musaf, Minchah and Ne'ilah.

(R. Yehudah): They bless during Shacharis and Musaf but not during Minchah or Ne'ilah.

(R. Yosi): They bless during Ne'ilah but not during Minchah.

The point of contention is as follows:

R. Meir holds that the Kohanim do not ordinarily bless during Minchah because they might be drunk, which is not a concern on these days.

R. Yehudah holds that since drunkenness is common on most days during the time of Minchah and Ne'ilah, the Rabanan decreed that the Kohanim should not bless at those times even on days where there is no such concern.

R. Yosi holds that this decree was only applied to Minchah, which occurs every day, but not to Ne'ilah.

(R. Yehudah citing Rav): The Halachah follows R. Meir.

(R. Yochanan): The people are accustomed to follow R. Meir.

(Rava): The custom is in accordance with R. Meir.

One who says the Halachah is like R. Meir means that he would lecture as such in public.

One who says the Minhag is like R. Meir means that he would tell people to do so if asked, but was not so confident as to publicly teach it.

One who says that they were accustomed to follow R. Meir means that we do not tell people to do so, but if they did, we do not make them start again..

(R. Nachman): The Halachah follows R. Yosi.

Conclusion: The Halachah does indeed follow R. Yosi.

Question: Why nowadays do the Kohanim bless the people at Minchah on a Ta'anis?

Answer: Since it is done close to sunset, it is like doing it during Ne'ilah.

Question: All agree that a Kohen may not bless while drunk - how do we know this?

Answer: The Torah juxtaposes the Kohen's blessing with the Nazir to teach that he is likewise forbidden to drink wine.

Question: Perhaps the Kohen should likewise be forbidden to eat grapeskins?

Answer: The Pasuk states that he should serve and bless; like someone who serves in the Beis ha'Mikdash, he may eat grapeskins.

Question: Perhaps, like someone who serves in the Beis ha'Mikdash, he may not perform if he is blemished?

Answer: He is linked to Nazir, who is not disqualified by being blemished.

Question: Why compare the Kohen to the one serving for the leniency of eating grapeskins rather than to the Nazir for the stringency of being forbidden to do so?

Answer: Since it is an Asmachta, it is d'Rabbanan, and therefore takes the lenient option.