The Tammuz is the initiation ceremony in which a djinn gains their powers. No djinn can use their power until he/she has fasted in the desert. The ceremony celebrates the rebirth of the djinn Tammuz and the eternal flame that burns within them.
The Tammuz is a ritual that is thousands of years old. This ritual is only practiced by the Marid by young Djinn as a rite of passage into adulthood. The main purpose of this ritual is to devise an appropriate focus word and to release their djinn powers.
According to the series, it is the rebirth of King Nimrod as Tammuz. The wife of Nimrod, Semiramis, gave birth shortly after the king's death, to a son named Tammuz. After delivery, she spent 40 days and nights in the wilderness to mourn. During the mourning she had a vision that her son was the rebirth of Nimrod.