The Islamic calendar was dated to start with the first new moon after the Prophet Muhammad and his followers migrated to Medina.

The calendar begins with the Prophet’s Flight as this is a key event in Islamic history for which all early followers could agree on the specific date. There was disagreement over the exact date of other events, such as the birth of the Prophet or when he first received the Divine message.

Known as the Hijra the Prophet Muhammad led his followers from Mecca to Medina, which at the time was called Yathrib, due to rumours of an assassination plot against him. Despite the migration taking place in 622, this year was only set as the start of the Islamic calendar by the Caliph Umar in 638 due to the pressing need to have a formalised dating system to improve administration. Until then the Muslim community had identified years according to a key event that took place within it – such as ‘the year of permission’.

While the Islamic calendar is linked to the Hijra, the actual start date is based on the beginning of the month of Muharram in the year of the Prophet’s arrival in Medina. This lunar month was already important to pre-Islamic Arabs and so served as a sensible demarcation, especially as it had been named by Allah in the Quran as one of the four sacred months.

16 July 622 was only identified on the Western Julian calendar during the medieval period. Muslim astronomers created a tabular Islamic calendar that they then projected backwards to identify the equivalent date on the Julian calendar. A tabular Islamic calendar relies on arithmetical rules to determine the length of the months, rather than astronomical calculations. Lunar observations are still used to specify the correct date of Islamic holidays and rituals.

On the 18th May 1291, the Crusader-controlled city of Acre was seized by the Muslim forces of the Mamluk Sultan Al-Ashraf Khalil.  The Siege of Acre, sometimes known as the Fall of Acre, marked the last attempt to exert Crusader influence in the Holy Land.

Acre had been under Christian control since it was besieged in 1191 during the Third Crusade, and had quickly become the capital of the Kingdom of Jerusalem.  With the rise of the Mamluk Sultanate in nearby Egypt in 1250, Crusader holdings became targets for conquest.

The spark for the attack on Acre was the suspected killing of a Muslim for an affair with the wife of a Christian.  This coincided with the arrival of over 1,600 poorly disciplined Italian reinforcements for the city, who allegedly pillaged nearby towns for supplies and killed a number of Muslims in the process.

These killings were cited by the Mamluks as reason to cancel a ten-year truce they had signed with the Crusaders.  Having amassed an army of many thousands, Sultan Khalil therefore began the siege on 5th April and within less than a month his forces had reached the city walls and begun to mine out the base of the walls and defensive towers.  These began to collapse on the 8th May, and a few days later the full infantry attack on the city began.  By nightfall on 18th May the Christians had been defeated, their leaders having either fled by boat or been killed in the fighting.

SOURCES:

http://www.historynet.com/third-crusade-siege-of-acre.htm

http://books.google.co.uk/books?id=mAcMAAAAYAAJ