The best days to fast during Dhul Hijjah are the first nine, as they are among the most blessed days of the Islamic year. Fasting on these days is a Sunnah of the Prophet Muhammad (PBUH), and a means of drawing closer to Allah.
It is prohibited (haram) to fast on the 10th of Dhul Hijjah, as it is the day of Eid al-Adha—a day of celebration and feasting. The Prophet (PBUH) said:
"He forbade fasting on the day of Eid al-Fitr and the day of al-Nahr "
(Hadith | Sahih al-Bukhari and Muslim)
If you are unable to fast all nine days, it is highly recommended to fast on at least the 9th of Dhul Hijjah, known as the Day of Arafah. This day holds immense significance, as Allah revealed in the Qur’an:
"This day I have perfected for you your religion and completed My favour upon you and have approved for you Islam as your religion."
(Qur’an | 5:3)
Fasting on the Day of Arafah carries great reward. The Prophet Muhammad (PBUH) said:
"It expiates the sins of the past year and the coming year."
(Hadith | Muslim)
However, those performing Hajj do not fast on this day, as they are considered travelers and are engaged in physically demanding rituals. Instead, their focus should be on du’a and remembrance of Allah.
Outside of Dhul Hijjah, it is also virtuous to fast regularly on Mondays and Thursdays, and during the White Days (13th, 14th, and 15th of each Islamic month), following the example of the Prophet (PBUH).