the reason for the success of the 1979 Iranian Revolution that overthrew Shah Mohammad Reza Pahlavi, although that explanation has also been questioned. Arba'ein in the Gregorian calendar While Arba'ein is always on nearly the same day (20 or 21 Safar) of the Islamic calendar, the date on the Gregorian calendar varies from year to year because of differences between the two calendars, since the Islamic calendar, the Hijri calendar (AH), is a lunar calendar and the Gregorian calendar is a solar calendar. Furthermore, the method used to determine when each Islamic month begins varies from country to country (see Islamic calendar). Arba'ein always falls 40 days after the Day of Ashura. The Day of Ashura, in turn, falls nine days after the first day of Muḥarram. Hence, Arba'ein falls 49 days after the first day of Muḥarram. This date is shown for a selection of years, according to the Umm al-Qura Calendar of Saudi Arabia, in the table below: Photographs See also List of largest peaceful gatherings in history List of casualties in Husayn's army at the Battle of Karbala Notes References Shia Islam Islamic festivals Mourning of Muharram Islamic terminology History of Shia Islam