Ask Rabbi Tuly: When is Purim?

Purim is celebrated in Jerusalem on Friday, March 22, 2019 and on Thursday, March 21 everywhere else. Why do some places celebrate the Jewish holiday of Purim on one day, while other cities observe it on another day? Rabbi Tuly Weisz explains based on Esther 9:17-19 that it all has to do with the biblical account that distinguishes between walled and un-walled cities.