Yosef Karo

“ And if Purim falls on Sunday, we advance the fast to Thursday. Rem"a: This fast is not obligatory, and one therefore can be lenient when necessary, eg. for one who is pregnant, nursing, or not dangerously sick. And even if one who has eye pain that is bothering them a lot should not fast. They can make it up at a later point. But other healthy people should not behave differently than the rest of the community (Hiddushei Agudah quoting the Mahzor Vitri) . And if Purim falls on Sunday, such that the fast is on the previous Thursday and there is a circumcision, one can eat for the circumcision ”