Muแธฅarram is the first month of the Islamic calendar, and one of the four sacred months of the year when warfare is forbidden. The tenth of Muharram is known as Ashura, an important day of commemoration in Islam. For Sunni Muslims, the day marks the parting of the Red Sea by Moses and the salvation of the Israelites, celebrated through supererogatory fasting and other acceptable expressions of joy. By contrast, Ashura is a day of mourning for Shia Muslims, who annually commemorate the death of Husayn ibn Ali, grandson of the Islamic prophet Muhammad and the third Shia imam. Husayn was killed, alongside most of his relatives and his small retinue, in the Battle of Karbala in 680 CE against the army of the Umayyad caliph Yazid ibn Mu'awiya. The Shia rituals span the first ten days of Muharram, culminating on Ashura with mourning processions in Shia cities. Also in Muharram, the Aqsa mosque in Jerusalem was initially set as the direction of prayer for early Muslims.