Laws of the Megillah
3. The fourteenth of Adar is Purim (for those dwelling in un-walled cities
(see Law 23) (1). If Purim falls on Sunday, the Fast [of Esther] should be
held on the previous Thursday. If a circumcision is celebrated that day
(Thursday), the meal should be held at night, but the "Sandik" (2) and the
father of the child may eat during the day (3). They do not have to
[compensate by] fasting on Friday. In contrast, a person who forgot and ate
[when the Fast of Esther is held on Thursday] should fast on Friday.
(1) Cities which were walled at the time of Joshua (who led the Israelites
into the Land of Israel), celebrate Purim on the fifteenth of Adar.
(2) The one who holds the baby during the circumcision.
(3) According to the Mishna Berura, the mother of the baby and the "Mohel"
(the one performing the circumcision) are also permitted to eat during the
day (Sha'ar Hatzion 686:15).