The Shi‘ite holy day/day of mourning and fasting, ‘Ashura, the 10th of Ramadan, bean at sundown and continues tomorrow. Many Sunnis also observe ‘Ashura, but not with the mourning rituals for Imam Husayn associated with Shi‘ism. There have already been sectarian attacks against Shi‘ite targets in Iraq. Egypt's small Shi‘ite community have announced their intention of praying tomorrow individually at the Sayyidna Hussein Mosque in Cairo, a shrine to Hussein founded during the (Shi‘ite) Fatimid Dynasty. In the past Salafis have sought to prevent the Shi‘ites from praying at the mosque.
My 2009 ‘Ashura post explained the background of the holy day.