
The letter that is crowned in the month of Tammuz is the letter Chet (ח) which means sin in Hebrew. Tammuz is the month of the sin of the golden calf, which resulted in Moses breaking the tablets that held the ten commandments. The day of the breaking of the tablets - the 17th of Tammuz - begins the three week period (Bein Hameitzarim) which commemorates the time leading to the destruction of the Holy Temple in Jerusalem. That period began with the breaking through of the wall surrounding the holy city and culminated in the destruction and burning of both Temples (in different eras) on the ninth of Av.
(You can read more here.) Chodesh Tov!