In Esther 9:20-22 and 9:26, we read about the end result of the whole book of Esther:
20 Then Mordecai recorded these events, and he sent letters to all the Jews who were in all the provinces of King Ahasuerus, both near and far, 21 obliging them to celebrate the fourteenth day of the month Adar, and the fifteenth day of the same month, annually, 22 because on those days the Jews rid themselves of their enemies, and it was a month which was turned for them from grief into joy, and from mourning into a holiday…
26 Therefore they called these days Purim after the name of Pur.
So, first off, let’s quickly review what actually happened in the book of Esther.
Esther, a Jew, was in a surprising situation where she was able to become queen of the Persian Empire (more accurately, the Achaemenid Empire). An important official in the king’s court then got angry with the Jews and convinced the king to issue a decree to massacre all the Jews in the Empire. But, Esther, with the help of her adopted father Mordecai, convinced the king to issue a new decree that allowed the Jews to fight back and preemptively kill all the people who planned to kill them. So, after the Jews had killed some 75,000 people, we read the above verses where they rejoice at having avoided their massacre.
But, there’s a weird language detail in verse 26. What do Purim and Pur mean?
Well, pur is just the Hebrew word for a “lot”, and purim is the plural, “lots”.
This is because the high official who wanted to kill the Jews had cast lots to determine what day the deed would be carried out.
Also, the Hebrew word pur might come from the Akkadian word puru, which means “a stone”. That would make sense because people would commonly use stones for the casting of lots (nowadays, we would typically use plastic, dice; or I guess flipping a coin works too).
And modern Jews still celebrate this holiday of Purim every year, typically in our month of March.
The book of Esther also explains how in the city of Susa (the capital of the Achaemenid Empire), the Jews killed their enemies for two days, while everywhere else the Jews only fought for one day. That’s why the Jews in the city of Susa would celebrate Purim on the 15th of Adar, but everywhere else Jews celebrate it on the 14th. Today, the city of Susa is no more, but the tradition of celebrating the “Shushan (of Susa) Purim” one day later still lives on in Jerusalem.
As a final note, the “King Ahasuerus” mentioned throughout the book is probably King Xerxes I, but we don’t know for sure. It gets complicated because the book only ever calls him by the Hebrew name Ahasuerus.