Just as there are days on which fasting is prescribed or recommended in Islam, there are also days on which fasting is either prohibited or considered inappropriate.
The days on which fasting is strictly prohibited are the days of ʿĪd. The Messenger of Allah ﷺ declared that fasting is not to be observed on the first day of ʿĪd al-Fiṭr and on the four days of ʿĪd al-Aḍḥā. This prohibition is due to the fact that the days of ʿĪd are days of eating, drinking, and rejoicing. ʿĪd al-Fiṭr is regarded as the collective breaking of the month-long fast, while ʿĪd al-Aḍḥā is a period of hospitality marked by the offering of sacrifices for the sake of Allah.
For pilgrims, fasting on the 8th of Dhū al-Ḥijjah (the Day of Tarwiyah) and the 9th of Dhū al-Ḥijjah (the Day of ʿArafah) is also considered inappropriate. The Messenger of Allah ﷺ prohibited those who are present at ʿArafāt from fasting, emphasizing that maintaining strength during the pilgrimage is more important.
Apart from these, fasting on certain days is considered makrūh (disliked) for various reasons. Fasting exclusively on Friday or Saturday, and particularly fasting only on the tenth day of Muḥarram (known as the Day of ʿĀshūrāʾ), is regarded as makrūh tanzīhan. Fasting on the last day of Shaʿbān (the Day of Doubt) is also makrūh, as it conflicts with the hadith prohibiting the anticipation of Ramaḍān by fasting one or two days in advance.
In addition, continuous fasting without breaking the fast between days (ṣawm al-wiṣāl) was prohibited by the Messenger of Allah ﷺ. Women who are experiencing menstruation or postnatal bleeding are exempt from fasting during these periods, and fasting at such times is unlawful. However, after Ramaḍān, they are required to make up the missed days of fasting.