The Hegirian calendar is based on the lunar cycle, which is why the Hegirian year is eleven days less than the Gregorian year. The Hegirian months are in no way related to the seasons based on the solar cycle, which means that Islamic holidays that fall in the same month each year can happen in different seasons. The hajj or the month of Ramadan can fall in summer or winter. The different Islamic occasions only fall on the same date once every thirty-three years, when the lunar cycle ends. There may be some small nuances between the different printed Islamic calendars for several reasons: (1) The lack of an international criterion that can confirm the birth of the new moon. (2) The use of various calculation methods for the perception of the crescent. (3) The bad weather that can strike the place where we proceed to this perception. This is why there is not yet a 100% accurate Islamic calendar. The ocular vision of the crescent - vision not based on calculations alone - is therefore essential to be able to determine some important occasions such as the first Ramadan and the days of the two feasts.