Reference: Fataawa as-Siyaam libni Jibreen – Page 101, Fatwa No.123
Question: What is the day of doubt [يوم الشك]? And is it permissible to fast on this day?
Response: The day of doubt is the 30th day of Sha’baan. It is not permissible to fast on this day, as there is a hadeeth on the authority of Salmaan (radhi-yAllaahu ‘anhu) who said:
«Whoever fasts the day which is doubtful then he has disobeyed Abal-Qaasim (Muhammed (sal-Allaahu ‘alayhi wa sallam))».