Esther 9:27 (ESV)

27 the Jews firmly obligated themselves and their offspring and all who joined them, that without fail they would keep these two days according to what was written and at the time appointed every year,

The Jews accepted this obligation not only for themselves, but also for their descendants and for all who joined them. After all, more and more people were joining themselves to the Jews, in response to the events they witnessed (Esther 8:17). This happened not only in the time of Esther, but also in earlier and later periods of biblical history (Exodus 12:38; Leviticus 24:10; Ruth 1:16–17; Isaiah 56:3; Matthew 23:15; Acts 2:10; Acts 6:5; Acts 13:43).