I fasted on Friday as I was making up for a day that I had missed. I intended to fast on Saturday as well but I had my menses on Friday evening. Was my fasting on Friday valid? I know that fasting on a Friday requires fasting on the day after; that is Saturday, otherwise the fasting would not be accepted.

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

If a person fasts on a Friday and then something prevents him from fasting on the day after; i.e. Saturday, then he is not sinful for this. This does not render the person’s fasting invalid because what is prohibited is to intentionally single out a Friday with fasting. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Do not single out Friday with fasting unless it is included in a plan of fasting that you regularly observe.” [Muslim]

Also, the prohibition that is stated in the Hadeeth refers to voluntary fasting. This means that the Muslim is not prohibited to single out a Friday with obligatory fasting, such as the fasting whereby one makes up for missed fasts in Ramadan and the fasting that one becomes committed to by vowing. This is stated by the author of Tuhfat Al-Muhtaaj and others.  

Allaah Knows best.