Today we begin the period known as בין המצרים, or "the three weeks." The Hebrew translation means, "between troubles." We begin with a fast day and end with a fast day.
This first fast is called שבעה עשר בתמוז, the 17th of Tammuz. On this day, five calamities occurred: The sin of the Golden Calf and the breaking of the two tablets, the daily sacrifice known as the תמיד, stopped being offered in the first Temple, ( the Kohanim could not find available sheep for the offerings.) the walls of Jerusalem were breached in the second Temple, Apustumus-the-wicked, burned the Torah, and an idol was placed in the Sanctuary.
We say סליחות today and עננו in Shmone Esrei. We start a period of mourning that increases as we remember the destruction of our two Temples. The peak of this mourning is on Tisha B'Av, when we act as if we are actually sitting Shiva.
We are promised that שבעה עשר בתמוז and תשעה באב will one day turn into days of celebration. May those days come soon.