Esther 2:16 (ESV)

16 And when Esther was taken to King Ahasuerus, into his royal palace, in the tenth month, which is the month of Tebeth, in the seventh year of his reign,

This event was of such importance that the author gives us its exact date. From extra-biblical accounts we know that this was the year in which Ahasuerus returned from his unsuccessful campaign against Greece and sought comfort in various love affairs. The fact of the war explains why four years had passed between the king’s dismissal of Vashti (Esther 1:3) and his marriage to Esther.