Purim is celebrated on the 14-15 of the Hebrew month Adar. As all Jewish holidays, Purim begins in the evening of Adar 13 and ends in the evening of Adar 14.
Shushan Purim: In Jerusalem that was surrounded by a wall at the time of Joshua, Purim is celebrated on the 15th of the month of Adar. This is in remembrance of the the continued clashes in the Persian capital city of Shushan.
Purim commemorates the annulment of a decree to kill all the Jews in the Persian empire. The story of Purim is told in the book of Esther of the Tanach.
Esther was the queen of Persian king Ahasuerus who ruled from 486 to 465 BC. The Greek name of king Ahasuerus is Xerxes.