Esther 9:21

9:21 to have them observe the fourteenth and the fifteenth day of the month of Adar each year

Esther 9:27

9:27 Therefore, because of the account found in this letter and what they had faced in this regard and what had happened to them, the Jews established as binding on themselves, their descendants, and all who joined their company that they should observe these two days without fail, just as written and at the appropriate time on an annual basis.