13th of Adar - Fast of Esther
Why is the fast on the day before Purim called Ta'anit Esther -- the Fast of Esther? (The three-day fast which Mordechai proclaimed upon the request of Esther took place during Pesach.)
Though Achashveirosh originally agreed to Haman's plan to kill all the Jews on the 13th of Adar, he later instructed the Jews to defend themselves against the Persian armies. According to halacha, people defending themselves against enemies are forbidden to fast, so as not to weaken their strength. (Shulchan Aruch, Orach Chaim 571:3)
Consequently, on the 13th of Adar it was forbidden for any Jew to fast. Esther lived in the King's palace, and thus did not fear the armies. Hence, she was the only person able to fast while the Jewish people defended themselves against the Persians. Therefore, the day is known as Ta'anit Esther.