1. The Hijri calendar was established by Umar after receiving a letter from the governor of Basra expressing confusion about the dating of letters without a standardized calendar system. 2. Umar gathered Islamic scholars and companions to discuss establishing an Islamic calendar, with four different opinions on when the first year should be. It was decided to set the start of the calendar from the Hijra event, so it is called the Hijri calendar. 3. The month of Muharram was chosen as the first month for three reasons - it was already the first month in the pre-Islamic lunar calendar, it comes after the Hajj pilgrimage, and the Hijra intention was set