Esther 9:26-27

26So the Jews called these days Purim, based on the word Pur. Therefore, because of everything that was said in this letter—both what they had seen and what had happened to them—

27the Jews established a tradition for themselves and their descendants and for anyone who would join them. The tradition was that a person should never fail to observe these two days every year, as they were described and at their appointed time.