The main purpose of the Book of Esther (written between 460 and 350 B.C.) is to celebrate the providence of God, especially in regard to His chosen people, Israel. Interestingly, it’s the only biblical book that does not mention God by name, yet throughout it reveals how God works behind the scenes to deliver the Jews in Persia. God uses Esther to help his people (Esther 4:14) and through her faithfulness, Haman is finally defeated and the Jews enjoy rest from their enemies.
The Book of Esther records the institution of the Feast of Purim and the obligation of its perpetual observation:
”Then Queen Esther, the daughter of Abihail, and Mordecai the Jew gave full written authority, confirming this second letter about Purim. Letters were sent to all the Jews, to the 127 provinces of the kingdom of Ahasuerus, in words of peace and truth, that these days of Purim should be observed at their appointed seasons, as Mordecai the Jew and Queen Esther obligated them, and as they had obligated themselves and their offspring, with regard to their fasts and their lamenting. The command of Queen Esther confirmed these practices of Purim and it was recorded in writing” (Esther 10:29-32).
The word Purim is derived from the Persian word pur (“lot”) and recalls how Haman, the enemy of the Jews, cast lots to determine the best day to carry our his plan to exterminate them (Esther 3:7). Of all the Jewish festivals, Purim is the most secular in flavor, and one of the most joyful. It is celebrated on the fourteenth of Adar (February/March), preceded by a day of fasting.
It is read at the Feast of Purim to commemorate the great deliverance of the Jewish nation brought about by God through Esther. Jews today still read Esther during Purim.
So what does it have to do with Christians?
The story of Esther is a lesson to Christians to trust God and allow God to use us in the outworking of his divine purpose. Christians don't need to celebrate Purim but they do need to pay heed to the history of God's dealings with his chosen people. It's a lesson in how God is Sovereign, and how worldly rulers can't have it all their own way. We learn from the mistakes of those who oppose God and the victories of those who trust God
Sources: New International Version and English Standard Version Study Bible Notes.