Eid al-Adha, the most important day in the Muslim calendar falls this year on Monday 12th September. Literally "the festival of sacrifice" the day commemorates the story of Ibrahim (Abraham) and Isaac which is also to be found in the Christian bible and the Jewish scriptures. Abraham was ready to sacrifice his son Isaac in obedience to God but was only being tested -- the boy was spared and a sheep sacrificed instead once he showed his faith. Eid el-Adha marks the end of the pilgrimage to Mecca. There is another Eid, called Eid al-Fitr, which marks the end of the fast of Ramadan earlier in the year. Muslim festivals follow the lunar calendar which means they fall on different dates every year.