The Islamic calendar, also known as the Hijri calendar, is a lunar calendar. It is based on the Qur'an and is the official calendar in many Gulf countries.
The Islamic calendar months start when the lunar crescent is first seen by human eyes after a new moon, so reliable Islamic calendars cannot be printed in advance. There are 12 months that comprise the Islamic calendar, with one of the most important months being Ramadan. The years on an Islamic calendar are counted since the Hijra, or Mohammed's emigration to Medina in 622 AD. Year 1 started on July 16th, 622.
Saudi Arabia only relies on a visual sighting of the crescent moon for religious purposes. For civil purposes, they base their calendar on a mathematically-calculated astronomical moon. Other Muslim countries only use the Islamic calendar for religious purposes, and for all other purposes turn to the Gregorian calendar.