Today is Shiva Assar BeTammuz (the 17th Day of the Month of Tammuz)
The five things that happened on this Day in Jewish History. This is why we fast

1. The First set of Tablets of the 10 Commandments were broken – due to the sin of the Golden Calf
2. The Walls of the City of Jerusalem were Breached in the battle against the Romans
3. A Sefer Torah was Burned by Apostemus
4. An Idol was Placed in the Beit Hamikdash / Holy Temple
5. The daily offering in the Beit Hamikdash was stopped

This starts the period of the three weeks where we observe certain aspects of mourning for the difficult occurrences.

One who mourns over Jerusalem will merit to see its rebuilding.

It should be rebuilt soon, in our days. Amen.