Is it permissible to fast on Saturday or Sunday individually if it is not during the month of Ramadan?

It is disliked to fast Saturday or Sunday individually. The proof for the dislike of fasting Saturday is what Ahmad and Abu Dawud narrated from As-Samma' bint Busr: "Do not fast on Saturday except for an obligatory fast. And if one of you finds nothing but a grape vine or the bark of a tree, let him break his fast with it." As for the dislike of fasting Sunday, it is due to what Ibn Khuzaymah narrated from Umm Salamah that the Prophet, peace and blessings be upon him, used to fast Saturday and Sunday and say: "Indeed, they are days of 'Eid for the polytheists, and I wish to differ from them." The intention is to fast Sunday with a day before it or after it, unless it coincides with a day one customarily fasted. The majority of scholars interpreted the prohibition as dislike, citing the hadith about the fasting of Dawud, peace be upon him. This ruling also includes fasting Friday individually.