Esther and the Jews fasted for three days, and Esther successfully approached the king to appeal for the deliverance of the Jewish people (for details, see the topic on Purim).
The Modern Observance
Today, Jews remember the fast called by Queen Esther on the day before the Jewish festival of Purim. Unlike the original fast, however, the modern observance is not observed for three days, nor is it observed on the same dates (from Nisan 14-16, immediately after Mordechai was informed of Haman's decree). Instead, the Fast of Esther is observed on Adar 13, the day that the nation of Israel was supposed to be destroyed according to Haman's evil decree -- but also the very day in which Israel successfully repelled her enemies: 
And so, on the thirteenth day of the twelfth month -- that is, the month of Adar -- when the king's command and decree were to be executed, the very day on which the enemies of the Jews had expected to get them in their power, the opposite happened, and the Jews got their enemies in their power. (Esther 9:1) 
On the thirteenth day of the month Adar; and on the fourteenth day of the same rested they, and made it a day of feasting and gladness. (Esther 9:17)
Why is the fast observed for only day on Adar 13th? There are various reasons given in the Jewish tradition, but the main reasons are that fasting is not permitted during the month of Nissan, and the Torah gives precedent for fasting before warfare. The aim of the fast is to affirm that a Israel does not prevail by physical or military strength, but only by lifting her eyes heavenward in prayer so that Divine Mercy might prevail in battle. However, some Jews will fast an additional two days following Purim.
Fast of Esther Customs (Minhagim)
- The fast begins at the break of dawn and ends after the Megillah (scroll) of Esther is read that evening in the synagogue. If the Fast of Esther falls on Shabbat, the fast is observed on the Thursday before (since it is forbidden to fast on Shabbat).
- Selichot (penitential prayers) and Avinu Malkeinu (Our Father, Our King) are recited during synagogue prayer services on Adar 13.
- At the afternoon service the Thirteen Attributes of God's mercy (Exodus 32:11) are recited from the Torah three times. It is also customary make a small donation for the poor (made in memory of the half-shekel given by Israel when the temple still stood) at this time.
- The Aneinu prayer ("Answer us") asking for special forgiveness is added to the morning and afternoon services by the prayer leader.
- Since there are leaps years on the Jewish calendar, the fast of Esther (as well as Purim) may occur on Adar II (rather than Adar I).
- Ta'anit Esther is a time of anticipating God's deliverance for His people. Though it is a fast (tzom), it is not a sad time, but rather a time to prepare to behold the victory of the Lord, Adonai Tzeva'ot, on behalf of His people.
- Prophetic Significance: Haman is a type of Anti-Christ (or satan) who will rise up in the last days to attempt (again) to destroy the nation of Israel. The Jewish people will again fast and prepare for God's ultimate deliverance, but this deliverance will clearly come to them at the pierced hands of the Mashiach Yeshua, the true King of Israel. There will be great rejoicing following this tribulation.
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