Thul-Hijjah is the twelfth and last month of the Hijri calendar. It is one of the Sacred Months mentioned in the Quran [Quran 9:36]. These months are specifically: Rajab (7th), Thul-Qa’dah (11th), Thul-Hijjah (12th), and Muharram (1st). Such is the way of Allah in His creation. Of all places, He Almighty chose Makkah and preferred it over others. And of all people, He Almighty chose Muhammad . And of the 12 lunar months, He Almighty chose four to be sacred.
Wronging others in the sacred months is a greater sin than wronging them in other months. Before Islam, Arabs would refrain from fighting during these months, to the extent that if one were to meet the one who killed his father or brother during these months, he would not harm him. This custom of theirs was a remnant of the way of Ibraaheem . Thus it had its roots in revelation. So when Islam came, it reestablished the sacredness of these months, although it lifted the ban on warfare during these months.
1) Fasting the Days of Hajj
As for fasting the first 9 days of Thul-Hijjah, the Prophet was not reported to have strictly adhered to it. However, it is good for a Muslim to fast most or all of these days due to the fact that fasting falls under the category of “good deeds.” Yet one should pay heed, as well, to the fact that the good deeds to be performed therein are not restricted to fasting.
2) Saying Allahu Akbar
Rather, one ought to perform a variety of good deeds during these days—especially giving charity, being good to others, recitation of Quran, uttering remembrances of Allah, most particularly repeating, "Allahu Akbar." It was reported by Al-Bukhari that Ibn ‘Umar and Abu Hurayrah used to go out to the market places on the first 10 days of Thul-Hijjah saying “Allahu Akbar” repeatedly, and the people would join them.
3) Fasting the Day of 'Arafah
Fasting the Day of 'Arafah is very important. Every Muslim capable of fasting on that day should do so due to its great benefit. The Prophet said: “Fasting on the Day of 'Arafah expiates the sins of two years, the past one and the coming one.” [Muslim] The Prophet also said: “There is no day on which Allah, Exalted and Most High, frees people from the Fire [of Hell] as He does on the Day 'Arafah.”
4) The 'Eed of Sacrifice
Thul-Hijjah is the month of 'Eed al-Adh-ha, the Festival of the Ritual Sacrifice, which lasts four days. The 10th day of Thul-Hijjah is the first day. It is also known as the Day of Slaughter (Yawm an-Nahr), for on it the sacrifice of cattle begins. On this day all Muslims, old and young, women and men, are recommended to attend the 'Eed prayer, and fasting on it is prohibited.
5) The Three Days of Tashreeq
Following 'Eed Day are the three Days of Tashreeq. Tashreeq means to expose something to the sunlight, and these days have this name because they were the days people would expose the butchered meat of their sacrifices to the sun after salting them in order to make either beef, lamb, or camel jerky. Fasting any of these three days (and the previous Day of 'Eed) is forbidden due to the statement of the Prophet : "The days of Tashreeq are days of eating, drinking, and mentioning Allah.”
6) Lessons in Reflection
There are lessons to be learned from the designation of these 10 days for exerting oneself in good deed.
a) We learn that among numerous good works, it is important not merely to do anything that is good in and of itself, but rather we are further to do the good that is appropriate for the time. Thus, we exert ourselves in one way in Ramadan and another in Thul-Hijjah.
b) These days are a blessing and an energizer for doing good works throughout the year.
c) They are a reminder that we have limited days in this life. Like the days of Thul-Hijjah come and go, the days of life come and go so we should utilize them in the highest and best way possible.
d) These days also make a way for those who feel that they did not perform to the optimum level they had hoped for in Ramadan to make up for any shortcomings which teaches one always to look to the future. Thus, if one finds that one has slackened in one's worship, good deeds, vigilance, or faith, rather than be burdened with guilt and bad feeling, one learns to strive with each new day.