Are we allowed to replace fast on saturdays? I have been told that jews normally fast on saturdays. I dont know how true is that.
According to Fatawa Shami, Bahr ur Raiq and Tahtawi it is makrooh to fix Saturday as a fasting day. But if someone fast on Friday and Saturday or Saturday is found on the 13th, 14th or 15th of the lunar calendar, then it is no more makrooh and hence permissible. If someone fast immediately after eid the 6 recommended fasts of Shawaal, then it will no more be makrooh to fast on Saturday (Ahmad). The hadith of Abu Dawood on the fact that it is the feast day for the jews stipulated this fatwa. On the same basis some ulamas disliked and categorised makrooh the fast of Saturday and Sunday. In any case if someone fasted on a Saturday, the fast would be valid, but the action is disliked and makrooh. The day of Friday also should not be fixed as a fasting day unless you fast one day before and one day after. (Tabrani)