It has been the custom of the People of Israel since the days of the Geonites to fast on the thirteenth day of the month of Adar, in remembrance of the days that Esther fasted, prior to her entry into the palace of King Ahasuerus, trying to annul the decree of extermination that hung over the Jews (Book of Esther 4:16) and in remembrance of the fast that the People of Israel carried out on the thirteenth day of Adar of that year.

The evil Haman's decree stipulated that on the thirteenth day of the month of Adar, all Jews would be killed, destroyed, and eliminated, from children, women, and the elderly, and their property plundered. By virtue of the miracle of Purim, the hanging of Haman, and the accession of Esther and Mordechai, King Ahasuerus sent a royal letter throughout the empire, in which he authorized the Jews to defend themselves against their attackers on the thirteenth of Adar and eliminate their persecutors. However, the first decree of extermination was still in place, since any edict bearing the signature and seal of the royal ring could not be annulled, allowing the enemies of the People of Israel to carry out their plans. In short, the thirteenth of Adar was the day on which the enemies of the Jews were permitted to attack and exterminate the Jews, while the Jews were permitted to defend themselves against their attackers.
Although Mordechai was already viceroy, the danger to the Jews still loomed, and it was still necessary for the Eternal to have mercy on us so that we could defeat our enemies on the battlefield. Therefore, the Jews who could not or did not need to fight performed teshuvah (repentance and return to the Creator) and fasted the entire thirteenth day of Adar, as tradition dictates, that in times of misfortune, it is necessary to return to the Creator. There is no better teshuvah than one accompanied by fasting, since it refines the materiality of man and returns his spirituality to a central and essential aspect of his person.
Thus, the People of Israel customarily fast on the thirteenth day of Adar, in remembrance of that fast. Furthermore, we still have enemies and persecutors who seek to exterminate us, so we still need to fast and do teshuvah anew each year.

From a ritual perspective, the Fast of Esther is lighter than the other minor fasts, since the latter were decreed by the prophets, while the Fast of Esther is the result of popular practice over the years. However, for practical purposes, there are almost no differences between the rules of the various fasts mentioned.

The religious services and Torah readings for the Fast of Esther on Shacharit and Mincha are identical to those for the other minor fasts, however, since it falls on the eve of Purim, "Nefilat Appaim" is not recited at Mincha (Mishnah Berurah 131:33). According to the Ashkenazi tradition, which customarily recites the "Avinu Malkenu" prayer on Shacharit and Mincha on public fast days, it is omitted for Mincha during the Fast of Esther, due to its proximity to Purim. When the thirteenth of Adar falls on Shabbat, the fast is brought forward to Thursday, so it is not held concurrently with Purim, and thus the Mincha service is similar to that of the other minor fasts.
(Pninei Halacha, Zmanim, Purim)

*This year the Fast of Esther will fall on the day Wednesday, March 16, and fasting is observed from dawn until the stars come out.

The Breslev Garden wishes you a Happy Purim!