Esther 9:27-28

the Jews throughout the realm agreed to inaugurate this tradition and to pass it on to their descendants and to all who became Jews. They declared they would never fail to celebrate these two prescribed days at the appointed time each year. These days would be remembered and kept from generation to generation and celebrated by every family throughout the provinces and cities of the empire. This Festival of Purim would never cease to be celebrated among the Jews, nor would the memory of what happened ever die out among their descendants.