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 persecutions 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 a 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 at different days based on their own sightings. The calendar is called Hijri calendar. The Arabic word Hijrah means emigration.
The Islamic calendar contains 12 months that are based on the motion of the moon, and because 12 lunar months is 12 x 29.53=354.36 days, the Islamic calendar is consistently shorter (11 Days) than a solar year, and therefore it shifts with respect to the Gregorian calendar.
The Islamic calendar is the official calendar in countries around the Gulf, especially Saudi Arabia. But other Muslim countries use the Gregorian calendar for civil purposes and turn to the Islamic calendar for religious purposes.
The Names of the months in the Islamic or Hijri Calendar: