We are obligated to fast on the seventeenth of Tammuz as it states:

“Thus said Hashem, Master of Legions, ‘The fast of the fourth [month], the fast of the fifth and the fast of the seventh and the fast of the tenth...’” (Zechariah 8:19)

teenth of Tammuz, which is the fourth month. “The fast of the fifth” refers to the ninth of Av, the fifth month of the year. “The fast of the seventh” is the Fast of Gedaliah which occurs in the seventh month (third of Tishrei). “The fast of the tenth” refers to the tenth of Tevet, the tenth month. These are all days of fasting and mourning. Those who are required to fast (men and women) may not transgress this obligation.

There are five reasons for fasting on the seventeenth of Tammuz:

1. The Luhot were broken on the seventeenth of Tammuz. It happened as follows: After Bnai Yisrael received the Torah on the sixth of Sivan, Moshe went up to Mount Sinai the following day — the seventh of Sivan. He assured the people that he would return forty days later. The people mistakenly thought that the day he ascended the mountain counted as the first day of the forty. Moshe meant that he would be away for a complete forty days and forty nights and therefore would return on the seventeenth of Tammuz, while the people thought it would be on the sixteenth of Tammuz. When Moshe did not return as promised according to Bnai Yisrael’s reckoning, they became very frightened. The Satan told them that Moshe had died. As a replacement for Moshe they erected a golden calf.

Moshe returned the next morning carrying the luhot. To his dismay, he saw the people dancing around the calf. The weight of the luhot was immense, but since they were the luhot of Hashem, Moshe was miraculously able to carry them. When he saw the people sinning with the calf, the letters of the luhot flew up to Heaven. As a result, the luhot became incredibly heavy and Moshe could no longer hold them — he cast them from his hands and they shattered! This was a tremendous tragedy — these luhot were the handiwork of Hashem Himself. As a consequence of the sin of the golden calf the Jewish people no longer were worthy to receive these luhot. 

The second set of luhot were fashioned by Moshe and Hashem inscribed the Ten Commandments upon them. This difference indicated the diminished spiritual status of Bnai Yisrael. Had the nation merited the first luhot, we never would have been subjugated to other nations, nor would there have been death in this world.

2. The Korban Tamid, the daily sacrificial offering was abolished on the seventeenth of Tammuz.

In the time of the Temple, every day a lamb was offered in the morning and a second lamb in the afternoon to atone for Bnai Yisrael’s transgressions. Shortly before the destruction of the first Temple (which occurred on the ninth of Av), the Babylonians breached the walls of Jerusalem on the ninth of Tammuz. From that time on, they were unable to obtain the sacrificial animals from the city, and they used the animals they had in the Temple. On the seventeenth of Tammuz there were no more sacrificial animals left in the Temple, and they were no longer able to offer the daily Korban Tamid.

Likewise, during the time of the Second Temple, the Korban Tamid was abolished on this day. Early during the Roman rule over Judea, the Romans surrounded Jerusalem, and they were unable to obtain sacrificial animals from outside the city. At first the Jews lowered chests filled with gold over the walls and the Romans exchanged them for two lambs. One day, however, a wicked old man informed the Romans that as long as the Jews could bring their daily offerings in the Temple the Romans will not be able to conquer them. He suggested exchanging the gold for pigs instead of lambs. The plan succeeded and on the seventeenth of Tammuz, the Korban Tamid was not offered. The Romans entered Jerusalem, killed many Jews and ruled over them until the destruction of the Temple.

3. In the time of the second Temple the walls of Jerusalem were breached on the seventeenth of Tammuz, by the Romans after a siege of several years. (The walls were breached on the ninth of Tammuz in the time of the first Temple, but we fast on the seventeenth, because the second hurban effects us more to this very day.)

4. On the seventeenth of Tammuz, Apostomos burned the Torah. Tragically, the day is considered “suitable” for such a travesty because the luhot were shattered on the seventeenth of Tammuz.

5. An idol was placed in the Temple on the seventeenth of Tammuz. There are those who say it was in the image of King Menashe and others maintain it was an image of Apostomos.

The Rambam states, “We fast on these days to commemorate the calamities that occurred and to awaken our hearts and follow the path of repentance. They serve as a reminder of the bad deeds of our forefathers, as well as our bad deeds, which are similar to theirs, all of which caused them and us so much anguish. By remembering we will repent and improve our ways.” Therefore, during these days we must each examine our actions and remedy whatever is wrong. The most important aspect of these fast days is repentance and not the actual fasting — the fasting prepares us to repent, and this is what must be uppermost in our minds and hearts. Those individuals who engage in idle things are missing the point of the fast — they are focusing on the secondary aspect and ignoring the essential. ('משנה ברורה תקמ“ט, א’)