 After the Iranian Revolution, Ayatollah Ruhollah Khomeini endeavored to bridge the gap between Shiites and Sunnis by declaring it permissible for Twelvers to pray behind Sunni imams and by forbidding criticizing the Caliphs who preceded Ali—an issue that had caused much animosity between the two groups.In addition, he designated the period of Prophet's Birthday celebrations from 12th to the 17th of Rabi Al-Awwal as the "Islamic Unity Week", there being a gap in the dates of when Shiites and Sunnis celebrate Muhammad's birthday.Following this period, Sunni–Shia strife has seen a major upturn, particularly in Iraq and Pakistan, leading to thousands of deaths.