Gemara

Mishnah: In the following months, messengers went out to inform people which day was fixed to be Rosh Chodesh (so they will know when are Yomim Tovim, Chanukah, Purim and Tish'ah b'Av) - Nisan, Av,...

Question: Why didn't messengers go out to inform people which days were fixed to be Rosh Chodesh Teves and Rosh Chodesh Tamuz (so they will know when are the fasts of the 10th and 17th)?

Question: "Tzom ha'Revi'i v'Tzom ha'Chamishi v'Tzom ha'Shevi'i v'Tzom ha'Asiri Yihyeh l'Veis Yehudah l'Sason ul'Simchah" - how can they be fasts and also days of Simchah?

Answer (Rav Chana bar Bizna): When there is Shalom, they are for Simchah. When not, they are fasts.

Answer (Rav Papa): When there is Shalom, they are for Simchah. When there are royal decrees (against Torah), they are fasts. When there is neither Shalom nor royal decrees, they are optional fasts.

Question: If so, the same should apply to Tish'ah b'Av!

Answer (Rav Papa): Tish'ah b'Av is more stringent, because several tragedies occurred that day;

On Tish'ah b'Av Bayis Rishon and Bayis Sheni were destroyed, Beitar was captured, and Yerushalayim was plowed.

Megilah 5a - R. Eliezer: Rebbi openly bathed on the 17th of Tamuz. He tried to uproot Tish'ah b'Av, but the other Chachamim disagreed.

Pesachim 54b - Shmuel: In Bavel (we need not fast for rain), the only Ta'anis Tzibur (with all the stringencies) is Tishah b'Av.

Rava and R. Yochanan: Pregnant and nursing women fast and complete the fast on Tishah b'Av, just like they do on Yom Kipur. Bein ha'Shemashos is forbidden.

Question: But R. Yochanan said that Tishah b'Av is unlike a Ta'anis Tzibur!

Answer (Rav Papa): It is unlike the first fasts of the Tzibur (due to a lack of rain. Then, one may eat the previous night). Rather, it is like the latter fasts.

Rishonim

Rambam (Hilchos Ta'anis 5:1): There are days on which all of Yisrael fast because of tragedies that occurred on them, to arouse the heart to Teshuvah.

Rambam (4): Four fasts are explicit in Nevi'im. "Tzom ha'Revi'i v'Tzom ha'Chamishi v'Tzom ha'Shevi'i v'Tzom ha'Asiri" refers to the 17th of Tamuz, the third of Tishrei, Tish'ah b'Av, and the tenth of Teves.

Rambam (5): If one of these fasts occurred on Shabbos, we postpone it until after Shabbos. If it occurs on Erev Shabbos, we fast that day. On all of them one may eat and drink the night before, except for Tish'ah b'Av.

Magid Mishnah: The Rambam says that Yisrael have the custom to fast at these times because the Gemara says that when there is neither Shalom nor royal decrees, they are optional fasts.

Note: The Rambam wrote (Halachah 4) that the four fasts are explicit in Nevi'im; this connotes that they are obligatory. The Magid Mishnah's text of the beginning of Halachah 5 (Yisrael have the custom to fast Bahen (on these fasts) forced him to say otherwise. According to our text, Halachah 5 begins 'Nowadays, Yisrael have the custom to fast on the 13th of Adar.'

Rosh (1:6): The Tzibur can decide whether or not it wants to fast. An individual should not separate from the Tzibur, unless the Tzibur decided not to fast.

Ran (DH u'Makshinan and DH ud'Amrinan): Since the three fasts (other than Tish'ah b'Av) are not obligatory, we do not bother to send messengers for them. Even if people are accustomed to fast, this does not make it obligatory. The Gemara asked why messengers are not sent for the fasts of Teves and Tamuz, implying that the custom was to fast then, and it answered that they are not obligatory. We cannot say that at first they were optional and later they became obligatory, for if so, messengers should be sent! However, the Ramban says that if all of Yisrael wanted, they could decide not to fast. Nowadays, Yisrael desired and became accustomed to fast. Therefore, an individual may not breach their fence. All the more so, nowadays that there are decrees and no Shalom, the verse obligates us to fast. It is an enactment of Nevi'im.

Beis Yosef (OC 550 DH Parich): Rashi defines Shalom to be autonomy for Yisrael. The Ramban and Tur say that it means when the Mikdash stands.

Ran (DH v'Chosav): The Ramban also says that letter of the law, all four fasts should be no more lenient than fasts that Beis Din decrees over lack of rain. It should be forbidden to bathe, anoint, wear shoes, have marital relations, or eat the night before. Also, Chachamim enact like mid'Oraisa law, i.e. Yom Kipur. A Tosefta teaches that when we decree fasts on the Tzibur beginning before nightfall, also Melachah and bathing are forbidden. Rebbi bathed on the 17th of Tamuz to uproot it, since there were no decrees in his day. Nowadays the custom is to fast without the other stringencies. The fasts depend on the desire of Yisrael, and this is what they chose. However, the original enactment was with the stringencies.

Question (Tosfos Megilah 5b DH v'Rochatz): Why does it say that Rebbi bathed on the 17th of Tamuz? When there are no decrees and no Shalom, even fasting is optional!

Answer (Tosfos): Just like our ancestors accepted to fast on these days, presumably they had already accepted to do so.

Poskim

Shulchan Aruch (OC 550:1): Everyone is obligated to fast these four fasts. It is forbidden to breach the enactment.

Kaf ha'Chayim (549:7): The Maharil said that if one ate by mistake, he should fast Monday and Thursday and Monday to compensate. Many disagree and say that he must fast the rest of the day, but need not fast another day to compensate, unless he wants to atone for eating on the fast. The Rema (568:1) says that people are stringent to compensate. It seems that if this is difficult, one may give Tzedakah instead.

Rema: However, pregnant and nursing women who are in great pain should not fast. Even if they are not in great pain they are not obligated to fast. The custom is that they fast. However, on Tish'ah b'Av they are obligated to complete the fast.

Source (Gra DH Mihu): Pesachim 54b says that pregnant and nursing women fast and complete the fast on Tishah b'Av. This implies that they do not fast on the other three fasts.

Kaf ha'Chayim (4): Yisrael accepted these fasts with the leniency to eat the night before, and to exempt pregnant and nursing women.

Mishnah Berurah (3): A woman is considered pregnant from when the fetus shows, just like regarding Vestos. However, perhaps she is considered pregnant after 40 days from conception if she is in pain. Within 40 days she is like regular women, unless she is in great pain. She should not be stringent at all if she feels weak.

Mishnah Berurah (4): One who is sick, even if he is not dangerously sick, should not be stringent to fast.

Kaf ha'Chayim (6): If the fast is very hard for weak or old people, they are exempt. An old weak person should not fast at all. Even on Yom Kipur, we give him to eat.

Mishnah Berurah (5): We need not train children to fast even part of the day, even a boy of 12. However, if a child is mature enough to mourn, it is proper to feed him only what he needs, such as bread and water or another simple food, so he should mourn with the Tzibur.

Shulchan Aruch (2): On these fasts, except for Tish'ah b'Av, one may bathe, anoint, wear shoes, and have marital relations. One may eat the night before.

Source (Gra DH Mihu): Pesachim 54b says that in Bavel, the only Ta'anis Tzibur (with all the stringencies) is Tishah b'Av. The Ramban says that all four fasts were originally enacted (to be optional) with the stringencies. The verse supports this; it equates all four. Rebbi uprooted the three fasts from the stringencies. Rebbi sought to do the same regarding Tish'ah b'Av, but failed. According to R. Aba, he only sought to do so when Tish'ah b'Av is on Shabbos.

Beis Yosef (DH u'Mah she'Chosav u'Mihu): R. Yo'el forbids bathing in hot water, but Tosfos and Avi ha'Ezri permit. The custom is to be lenient.

Question (Taz 1): If it is permitted to bathe, what was the Chidush that Rebbi bathed on the 17th of Tamuz? If this is the source that bathing is permitted, why did Tosfos ask why it says that Rebbi bathed?

Answer (Taz 1): The Ramban says that says that the original enactment was to fast with the stringencies. Presumably, in the days of Rebbi that was the custom. Rebbi wanted to uproot the entire fast. Tosfos asks, if so he should have eaten! Tosfos answers that indeed, he wanted to totally uproot it, but he himself accepted to fast without the other stringencies.

Bach (DH 11): If there are decrees, the fasts are obligatory with all the stringencies. Nowadays people do not bathe on these fasts.

Mishnah Berurah (6): These are permitted because the majority of people cannot fulfill them. Therefore, a Ba'al Nefesh should be stringent on all these fasts like on Tish'ah b'Av, except for not wearing shoes, lest people mock him. One should not be stringent about marital relations if his wife's Tevilah occurs that night. If the 10th of Teves is on Erev Shabbos, one should not be stringent about bathing in hot water. One should bathe to honor Shabbos.

Kaf ha'Chayim (13): Even the Bach permits bathing on cold water, or washing the hands, feet and face in hot water. One who is stringent about bathing should not smell spices.