❝The Sahabah would make up their missed fasts of Ramadan in the first ten days of Dhu-l Hijjah. This is because of the great virtue of these ten days.❞
— Shaykh Salih al-‘Usaimi (h)
Fasting the first 10 days of Dhu-l Hijjah is recommended, and the day of ‘Arafah is extra recommended for those not performing Hajj. It is permissible to make up missed fasts on the first 8 days of Dhu-l Hijjah, but the Day of ‘Arafah should be fasted separately.
Note: The scholars say the first ten days because they are always mentioned together in the Qur’an and Sunnah. Otherwise, it is well known that fasting on the 10th (‘Eid day) is not permissible.