Today, Thursday February 21st (Adar 11th) we commemorate the "Fast of Esther".
When Queen Esther was informed of Haman's plan to eliminate the Jewish people, she decided to approach King Aḥashverosh. This move was very risky. A suicidal mission.
No one, the Queen included, was allowed to request a meeting with the King. Queen Esther, of course, lived in the palace, but Persian Emperors were obsessed with their personal security, and rightly so: in 465 BCE Aḥashverosh himself was assassinated by one of his bodyguards, Artabanus. The Laws of the Persian empire said that anybody who would come near the King without being summoned by the King, even the Queen, should be immediately executed. Josephus explains that the King had guards to the sides of his throne with special axes to execute on the spot any trespasser. But Esther had no choice. There was nothing anyone else was able to do to gain access to the King and persuade him to stop Haman's decree. Esther, therefore, decided to risk her life in order to personally approach the King.
Before Esther embarked in her mission she asked every Jew to fast. Fasting -together with prayer- is what our Tora and our Rabbis instructed us to do in difficult circumstances.
At the request of Esther, every Jew in the Empire fasted for three consecutive days and prayed for Esther's success.
In remembrance of that fast, established by Esther, we do today the fast of Esther.
Normally, ta'anit Ester takes place one day before Purim, on the 13th of Adar. However, when Purim falls on Saturday night/Sunday like this year, the fast day is observed the previous Thursday, i.e., today.
Ta'anit Ester is considered a minor fast. Pregnant or nursing women do not do this fast. Also children, elders or anyone with even a minor medical condition is exempted from this fast, especially this year that the fast is moved to Thursday.
In NYC the fast begins at 5:37AM and ends at 5:58 PM