What are the days on which fasting is not permissible?

Question : What is the day on which fasting is not permissible? Is it Saturday or Friday? And why?

Answer : There are five days on which fasting is not permissible: 'Id al-Fitr and 'Id al-Adha, and the three days following 'Id al-Adha, which are the days of Tashriq. Fasting on the days of Tashriq is not permissible except for one performing tamattu' or qiran who did not find the sacrificial animal; in that case, he fasts three days during Hajj and fasts three more when he returns. Apart from these five days, fasting is permissible, whether on Saturday or Friday. However, fasting on Friday alone is disliked, though it is not forbidden; it is merely disliked.
If someone fasts on Friday, we tell him to fast on Saturday with it, or on Thursday with it, unless Friday happens to be a day on which fasting is recommended. In that case, there is no harm in fasting on it alone, just as if Friday happened to be the Day of 'Arafah. If a person wants to fast because it is the Day of 'Arafah, not because it is Friday, there is no blame on him for that, due to the generality of the saying of the Prophet (peace and blessings be upon him): .
As for Saturday, the scholars have differed on whether fasting on it alone is permissible or disliked. Some said it is permissible because the hadith prohibiting fasting on it is weak. Others said it is not permissible but rather disliked, so fasting on it alone is disliked. If he fasts on it along with Friday, or along with Sunday, there is no harm in that.