She replied, “No.” He asked, “Do you intend to fast tomorrow?” She replied, “No.” He said, “Then break your fast.” Narrated by al-Bukhari.

As for fasting on Friday, the compiler has set aside a separate chapter for it, namely the dislike of singling out Friday for fasting or its night for voluntary prayer. Friday is the weekly festival; it recurs every seven days—indeed, more than fifty times each year. Because it is a festival, the Prophet ﷺ forbade fasting on it; yet the prohibition here is one of dislike, not absolute unlawfulness, precisely because of its frequent occurrence. By contrast, the prohibition of fasting on the two Eids—Eid al-Adha and Eid al-Fitr—is strictly unlawful, for each of them comes only once a year: Eid al-Fitr once, and Eid al-Adha once.

Since Friday recurs regularly, its prohibition is lighter: it is merely disliked, and that dislike is removed if one adds to it a day before or a day after. For this reason the hadiths of Abu Hurayrah (may Allah be pleased with him) report that the Prophet ﷺ said, “Do not single out Friday for fasting, nor its night for standing in prayer.” However, if a person is not singling it out—such as someone who prays every night, and therefore prays on Thursday night, or someone who fasts every other day and Friday happens to be his fasting day—then there is no objection to fasting on Friday. Likewise, if Friday coincides with the Day of ʿArafah or the Day of ʿAshura, there is no harm in fasting it.