Zul Hijjah is the last month of the Islamic calendar. It is when the Hajj, the annual pilgrimage to Mecca is performed by Muslims. It is a month with special merits for the believer, particularly during the first ten days of Zul Hijjah, when Muslims performing the Hajj remember the trials faced by Prophet Abraham.
"Surely Abraham was an example, obedient to Allah, by nature upright, and he was not of the polytheists. He was grateful for Our bounties. We chose him and guided him unto a right path. We gave him good in this world, and in the next he will most surely be among the righteous." (Holy Qur'an 16:120-121)
One of the most challenging trial faced by Prophet Muhammad was when he was asked by Allah (God) to kill his own son. Prophet Abraham prepared to obey God's command, and to kill his cherished son, but just at the moment when he was about to kill his son, God stopped him and said his command had been fulfilled. It was Prophet Abraham's absolute love, faith and obedience to God above all else that mattered.
If one is not performing the Hajj, there are other ways to gain the merits of Zul Hijjah.
The Holy Prophet, Sallallaahu Alayhi Wasallam, has said:
"One fast during these days is equal to the fasting of one complete year, and the worship of one night during this period is equal to the worship in the "Lailatul-Qadr".
Although, one can fast during any of the first 9 days of Zul Hijjah, it is recommended to fast on the 9th day of Zul Hijjah. The 9th day of Zul Hijjah is called 'Yawmul- "Arafah' (The Day of 'Arafah). This is the date when the pigrims assemble on the plain of 'Arafat, six miles away from Mecca, where they perform the most essential part of the prescribed duties of hajj, namely, the 'Wuqoof of Arafat (the stay in 'Arafat). For those not performing hajj, it is Mustahab (desirable) to fast on this day according to their own calendar.
However, it should be noted that it is haram to fast on the 10th, 11th, 12th and 13th of Zul Hijjah.
Also, it is mentioned in a Hadith that the person who keeps three fasts in the sacred months, on Thursday, Friday and Saturday, he will receive the reward of making ibaadah for two years. That is, on the Day of Judgement it will be written in his book of deeds that he had made ibaadah for two years in return for these three days of fasting.
Please refer here, for Islamic Calender 2010.
