"[The Jews sent to the Kohanim and the prophets the following question:] Shall I cry on the fifth month (Av)? And shall I abstain like I have for these many years (since the first Beis ha'Mikdash was destroyed)?..."

(Zecharyah 7:3).

Hashem said [to the prophet Zecharyah]: the fast of the fourth month, the fast of the fifth month, the fast of the seventh month, and the fast of the tenth month shall be for the House of Judah for happiness and rejoicing, and for festival days, if they keep peace and truth, the Torah and peace"

(Zecharyah 8:19).

Since these fasts were originally given over by a prophet through a prophecy, they have the status of "Divrei Kabalah," which is a very strong status and very close to actual laws of a Torah obligation, and they are more stringent than Rabbinic enactments (Rosh Hashanah 19a).

The Gemara in Rosh Hashanah (18b) adds that Tisha b'Av is kept differently than the other three fasts. Because the day of Tisha b'Av witnessed the destruction of both the first and second Beis ha'Mikdash, therefore even in times when there are no decrees against the Jews, we still must fast on Tisha b'Av. The exact status of the fast of Tisha b'Av, though, is not clear. According to the Rashba in Rosh Hashanah (18b) and in Megilah (5b), Tisha b'Av is not called "Divrei Kabalah." That is, the custom to fast on Tisha b'Av because its tragedies were doubled is a later enactment which is a Rabbinic decree or even a custom that the Sages decided to accustom people to fasting.

However, the Ritva in Rosh Hashanah (18b) seems to say that the fast of Tisha b'Av that we observe is considered Divrei Kabalah; now that the Beis ha'Mikdash was destroyed twice, we have accepted upon ourselves to observe that day as a fast, and the original Divrei Kabalah included that when we accept upon ourselves to fast, we are obligated to fast. Therefore, the fast of Tisha b'Av today is considered Divrei Kabalah. This is also the opinion of the Tur (OC 500:54) and the Eshkol (2:5). The status of the other fasts that we observe today is, according to most authorities, certainly not Divrei Kabalah; we fast on Tzom Gedalyah, Asarah b'Teves, and the Seventeenth of Tamuz only because it became customary for us to fast, but there was never an official enactment that we must fast on those days. Therefore, those days are not as stringent as Tisha b'Av.

(a) Rav Shlomo Yosef Zevin, in Moadim ba'Halachah, answers with the words of the Rashba (Teshuvos 1:614). The Rashba explains that it is clear from the Rambam that it is a Torah prohibition to fast on Shabbos. The Rambam (Hilchos Nedarim) says that if a person makes an oath to fast on Shabbos, his oath is not binding because he is making an oath to violate something which is from the Torah. Apparently, fasting on Shabbos is a Torah prohibition, even though the positive commandment to have pleasure on Shabbos is only from Divrei Kabalah. If so, it is clear why we may not fast on Shabbos when Tisha b'Av falls on Shabbos, and we push off the fast until the following day.

Rav Zevin quotes the Shulchan Aruch ha'Rav who finds a source for the Mitzvah in the Torah not to fast on Shabbos from the verse which discusses eating Manna on Shabbos. The Torah says that "You must eat it today," and from there we see that a person may not fast on Shabbos.

(b) The Minchas Chinuch himself suggests a novel answer. Perhaps the Divrei Kabalah was only to fast a "fast of the fifth month," as the verse in Zecharyah says. There was no original obligation from Divrei Kabalah to fast on a specific day (such as the ninth of Av). Therefore, the obligation to fast specifically on the ninth of Av was a later enactment that was made after the Churban of the second Beis ha'Mikdash, and only has the status of a rabbinical decree. One fulfills the Divrei Kabalah, though, by fasting on any day of the fifth month. Therefore, the fast cannot override the obligation to eat on Shabbos, since the Divrei Kabalah will be fulfilled if we fast on the next day. It is only a rabbinical decree to fast specifically on the ninth of Av, and the Divrei Kabalah obligation to eat on Shabbos overrides the rabbinical decree to fast specifically on the ninth.

(a) The Rishonim prove this from the Gemara in Ta'anis (29b). The Gemara explains a contradiction in the verse. We know that the "fast of the fourth month" is the seventeenth of Tamuz. Yet we find in the verse that the ninth of Tamuz was the day that Bavel laid siege on Jerusalem before the destruction of the Beis ha'Mikdash. Why do we fast on the seventeenth because of the siege of Jerusalem, and not on the ninth?

The Gemara answers that during the destruction of the first Beis ha'Mikdash, the siege started on the ninth of the month, and during the destruction of the second Beis ha'Mikdash, the siege started on the seventeenth. If so, asks the Ritva, the fasts which were instituted originally by Divrei Kabalah after the destruction of the first Beis ha'Mikdash would have been instituted based on what happened during the destruction of the first Beis ha'Mikdash, and thus the fast for the siege would have been on the ninth of Tamuz. Why is it, then, that we can decide, after the destruction of the second Beis ha'Mikdash to fast on the seventeenth of Tamuz, in contradiction to the Divrei Kabalah? The Ritva suggests, therefore, that perhaps the obligation of Divrei Kabalah was only to fast during the fourth month, but not on any specific day of the month, and therefore we could choose later on what day in the month to fast.

This also appears to be the intention of the Ramban (Toras ha'Adam) and the Tur (end of OC 549) who say that even though during the time of the first Beis ha'Mikdash the siege started on the ninth of Tamuz, nevertheless we observe the fast on the seventeenth of Tamuz because the destruction of the second Beis ha'Mikdash is closer to us and more meaningful to us, because that is what caused us not to have the Beis ha'Mikdash. They conclude saying, "However, the fast of the fourth month covers the siege of the city and all of the tragedies that happened throughout the generations, including the siege at the time of the first Beis ha'Mikdash." How does this fast, on the seventeenth of the month, include the siege of the first Beis ha'Mikdash which happened on the ninth? Perhaps since it is in the same month as the second siege, and the original decree of Divrei Kabalah was to fast during that month, therefore by fasting during that month we are mourning for the first Beis ha'Mikdash as well.

(b) The Minchas Chinuch cites additional proofs. In Ta'anis (29a), Rebbi Yochanan says, "If I would have been there at the time that the first Beis ha'Mikdash was destroyed, I would have declared the fast to be on the tenth of Av, and not the ninth of Av, because the Beis ha'Mikdash was set afire towards the end of the ninth and it burned throughout the tenth of Av, and thus most of the destruction occurred during the tenth of Av." The Minchas Chinuch asks how can Rebbi Yochanan say that "I would have declared the fast to be on the tenth?" Divrei Kabalah means that the prophet told us when Hashem wants us to fast for these four fasts! How can Rebbi Yochanan go against the word of the prophet? It must be that the prophets only said in what month to fast, and the Sages instituted on which day to fast.

Perhaps, originally, when the Divrei Kabalah was originally given to us telling us that we were to fast on the four fast days, those laws that the prophet taught us about the fasts included that they are not to be fasted on Shabbos. The two Divrei Kabalah -- to eat on Shabbos and to fast on Tisha b'av -- do not contradict each other. The Divrei Kabalah that says we must have pleasure for the sake of Shabbos is a much earlier Divrei Kabalah, and when the prophets taught the later Divrei Kabalah at the time of the destruction of the Beis ha'Mikdash, we were told to fast but not on Shabbos, and to push it off a day if the fast falls on Shabbos.

We might ask, though, what logic is there to this? If there is nothing preventing it other than Divrei Kabalah, why would the prophecy tell us that we should not fast on Shabbos?

The answer might be that fasting on Shabbos would be counterproductive. What is the purpose of fasting for the destruction of the Beis ha'Mikdash? The Rambam (Hilchos Ta'anis) tells us that it is in order to feel our plight and to realize how much we are missing in order to arouse ourselves to do Teshuvah for our sins so that we will merit the rebuilding of the Beis ha'Mikdash. Perhaps on Shabbos, which is a day of inherent joy, a day referred to as a "taste of Olam ha'Ba," not being part of the routine of normal life, it is impossible to feel the loss of the Beis ha'Mikdash as much. It is a time when we are closer to Hashem, when we elevate ourselves to a more spiritual level, and therefore we will not feel the loss of the Beis ha'Mikdash as much as during the week. Therefore, the Divrei Kabalah may have been to push off the fast until after Shabbos and to specifically not fast on Shabbos, so that the fast should be more productive when we fast during the week so that we take to heart the loss of the Beis ha'Mikdash and realize how much we are missing and how far away we are from Hashem and Daven to Hashem with all of our heart that he should rebuild the Beis ha'Mikdash.