Forbidden to fast on תענית אסתר

 

The fast day of תענית אסתר* on the 13th of Adar is not mentioned in בבלי or ירושלמי. It was instituted by the Gaonim. (The first mention of תענית אסתר is in the שאילתות דרב אחאי גאון) Before they abolished the  מגילת תענית it was forbidden to fast on the 13th of Adar  because the day before or after the  Yomim Tovim mentioned in מגילת תענית (Purim Chanuka etc,) were also included  as the days forbidden to fast. The רא"ש quotes the ר' תם  "The only  סמך  (minor  source) for fasting is from the Gemoro Megilah 2a that says י"ג זמן קהלה לכל היא The ר' תם explains they gather to say Selichos because of the fast day". *not to be confused with the three days fast in Nissan mentioned in the Megillah.