If, G-d forbid, Moshiach has not arrived by then, we begin fasting at halachic dawn tomorrow, and continue to do so until nightfall. In London, this means that we will be fasting from 1:05 AM until 10:01 PM. The rules of fasting apply to anyone over bar or bas mitzvah age, excluding those who are ill, pregnant, elderly, or weak.
We fast on 17 Tammuz in commemoration of five events which took place. The list below, courtesy of Chabad.org, summarises them;
1. Moses broke the tablets when he saw the Jewish people worshipping the Golden Calf.
2. During the Babylonian siege of Jerusalem, the Jews were forced to cease offering the daily sacrifices due to the lack of sheep.
3. Apostomos burned the holy Torah.
4. An idol was placed in the Holy Temple.
5. The walls of Jerusalem were breached by the Romans, in 69 CE, after a lengthy siege.
Despite these tragic events, the fast day is an auspicious day for prayer. As we refrain from worldly pleasures, and pour our hearts out to G-d, we can only hope that he will listen and send Moshiach, speedily and in our days iyH!