When it is recommended to fast
The Prophet (peace and blessings be upon him) recommended that we fast the three "white days" of every lunar month, the day of 'Arafa, 'Ashura (10th of Muharram) and either the day preceding or following it, and the six days from the month of Shawwal, promising a great reward for the person who fasts on any of these days.
When it is unlawful to fast
It is unlawful to fast on the following:
- 'Eid ul-Fitr.
- 'Eid ul-Adha.
- Ayam al-Tashreeq (the three days following 'Eid ul-Adha).
Ayam al-Tashreeq
The Prophet (peace and blessings be upon him) called these the days of food and drink.