Muharram is the first month of the Islamic calendar. It is one of the four months of the year in which fighting is prohibited. Since the Islamic calendar is lunar, Muharram moves from year to year when compared with the Gregorian calendar. Muharram is so called because it was unlawful to fight during this month; the word is derived from the word ‘haram’ meaning forbidden. It is held to be the most sacred of all the months, excluding Ramadan. Some Muslims fast during these days. The tenth day of Muharram is called Yaumu-l 'Ashurah, meaning, ‘the tenth day’, and it is a day of voluntary fasting. Fasting differs among the Muslim groupings; mainstream Shia Muslims stop eating and drinking during sunlight hours but do not eat until the evening. Sunni Muslims also fast during Muharram and on either the ninth or the eleventh day, the choice of which additional day being at the discretion of the individual.