The Hijri calendar also called the Islamic calendar is based on the year prophet Muhammad (PBUH) and his fellow Muslims (known as Sahabah, the Companions) emigrated to Madinah in the year 622 C.E. (Christian Era). The emigration took place after thirteen years of persecution by the disbelievers of Makkah. By the command of God, the Prophet left the city with his companion Abu Bakr Siddique (R.A.) and escaped a death threat by the disbelievers. The event marks the beginning of the second phase of the Islamic movement. It is the phase when Madinah became the center of an Islamic state.
The Islamic calendar is lunar. Each month must begin with the evening when the new moon is sightable by the unaided naked eye. Muslims are obligated to sight the crescent in every country. Different countries may begin the year on different days based on their own sightings. The calendar is called Hijri calendar. The Arabic word Hijrah means emigration.