Purim is the annual celebration established to remember the days when the Jews gained relief from their enemies and their sorrow was turned to joy (Esther 9:20–22). It is called Purim because of “Pur” (the lot) that was cast in connection with Haman’s plan (Esther 9:24–26).
In Esther 9:20–28, Mordecai records what happened and sends letters to Jews across the empire, directing them to keep two days every year in Adar. These days mark the Jews’ relief from enemies and the turn from grief to joy. The practice includes feasting, sharing food with one another, and giving to the poor. This section comes after the narrated fighting and celebration in Esther 9, where the Jewish communities gain safety and rest in different places on different days. The story now shifts from events on the ground to how those events will be remembered.
Esther 9:20–28 explains how a one-time deliverance became a permanent, yearly remembrance. Mordecai writes an account and sends letters to Jewish communities across the empire, “near and far,” requiring the 14th and 15th of Adar to be kept every year.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Esther 9:20-22, then read the surrounding section so the answer stays connected to the passage's flow.
20Mordecai wrote these things, and sent letters to all the Jews who were in all the provinces of the king Ahasuerus, both near and far,
21to enjoin those who they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly,
22as the days in which the Jews had rest from their enemies, and the month which was turned to them from sorrow to gladness, and from mourning into a good day; that they should make them days of feasting and gladness, and of sending portions one to another, and gifts to the poor.