The Megillah states that the evil Haman cast lots to determine which day would be the most auspicious for him to carry out his evil machinations. We all know that the lots fell on the 13th of Adar. The little known trivia though, is the day that the lots were cast. It says explicitly in the Megillah that it was done in the month of Nissan, but it does not state the day. The Midrah brings that it was on the 13th of Nissan. Every day that the lot fell on, Haman disqualified because there was some merit for the Jewish nation on that day. Until he finally came to the month of Adar, which he felt was a good month because that is when Moshe Rabbeinu died. What he didn’t know was that Moshe Rabbeinu also was born in that month which served as a merit for the Jews.
Another thing that occurred on the 13th of Adar was that the plague of darkness started in Egypt. Haman thought this would be a propitious day as many Jews were killed in that plague.