Date: 2nd Century B.C.E.
The book of Esther, is inextricably bound up with the holiday of Purim. The book provides the raison d'etre for Purim; it supplies the etiology (story of its origin) for the holiday, authorizes its annual observance, and models how it is to be celebrated. Purim, the only biblical festival not mentioned in the Torah, needed a reason and an authorization. It gets its reason in the pseudohistorical tale of how the Jews were saved from their archenemy, and it gets its authorization in the letter of Mordecai (and Esther) that this book includes.