When it began:
During the Purim miracle, the fighting in the walled city of Shushan continued through the 14th day of Adar and they were done by the 15th day. In commemoration, all known walled cities, which today include Jerusalem and the old city of Tiberius, celebrate Purim on the 15th of Adar.
How it’s observed today:
All the commandments, celebrations and customs that we do on Purim, Jerusalemites and people living in the Old City of Tiberias do on Shushan Purim.