Reference: Fataawa Noor ‘alad-Darb – Question No.6142, Page 567, Volume 11
Question: My sister undertook a vow to fast on Friday, so is it permissible for her to fulfil her vow or not? And if it is not permissible, is she then required to make an expiation?
Response: We say to her that [when] she fasts on Friday, in addition to it, she should fast on Saturday or Thursday; and with that, she has fulfilled her vow in a [legally] acceptable manner. As for a person specifically singling out Friday to fast for no other reason, then the Prophet (sal-Allaahu ‘alayhi wa sallam) prohibited that, unless he was to fast a day before it or a day after it, in which case we say to this woman: fast on Friday, and [be sure to also] fast a day before it or a day after it.
 Saheeh al-Bukhaaree, No.1884