On the 13th of Adar, the Fast of Esther is observed in memory of the Fast observed by Mordechai and Esther and all Israel. On that very day, the enemies of the Jews had planned to subjugate and destroy them. 'The opposite, however, occurred and the Jews ruled over their enemies. The practice of fasting was observed by the people of Israel whenever they were faced by war. Thus Moshe Rabenu also fasted when he came to wage war against Amalek. The aim of the fast was to affirm that a man does not prevail by physical or military strength, but only by lifting his eyes heavenward in prayer so that Divine Mercy might give him the strength to prevail in battle. This then was the purpose of the fast observed by Israel at the time of Haman, when they gathered to defend themselves against those who sought to destroy them. And in memory of that Fast, a yearly Fast was fixed for generations on the same day. We are to recall thereby that God accepts each person's prayer and penitence in the hour of his trouble.
The acceptance of this Fast of the 13th of Adar on the, part of Israel for later generations, is alluded to in the Scroll of Esther: And as they accepted upon themselves and upon their children, the matters of their fastings and their cry (Esther 9).
The Fast is called by the name of Esther because it was she who first requested the observance of a fast, of Mordechai: 'Go and gather all the Jews who are found in Shushan and fast over me, and do not eat and do not drink three days, night and day; and I and my maidens will also fast thus. (ibid. 4).
The fast which we observe is nevertheless not observed for a three-day period, as was the case with the original Fast, nor is it observed on the same date. Originally the Fast was observed by Esther and the entire people of Israel on the 14th, 15th and 16th of Nisan, immediately after Mordechai was informed of Haman's decree and of the letter of annihilation which Haman wrote on the 13th of Nisan. Our Fast however, is observed on the 13th of Adar, in memory of the Fast observed by Israel on the day of their mobilization for war against the enemies. The Fast is nevertheless called by the name of Esther since it was she who first proposed its observance.
Others hold the view, that even our Fast is also primarily a memorial to the original three-day Fast observed by the Jews when the decree was announced. But since the Fast could not be permanently fixed for later years in its proper time (because fasting is not permitted during Nisan), the Sages therefore fixed it for the 13th of Adar - which was also a Fast day for the Jews, who then gathered to wage war against their enemies. And although the Fast of Esther is therefore a memorial to the original three days of fasting, the Rabbis were nevertheless lenient in fixing it for only one day.
In deference to this view, there are some who fast an additional three days; on Monday, Thursday and Monday after Purim. Others voluntarily fast the night as well as the day on the 13th of Adar, since the original three-day Fast was observed night and day.
Since the Fast of Esther is not one of the four Fast days which are specifically mentioned in the Prophetic Writings, it is observed with greater leniency than the other Fast days. Pregnant women, nursing mothers, as well as others of generally weak health, (who would suffer by fasting) do not fast therein. The additional penitential prayers, and the Torah Reading, which are prescribed for the other Fast days are also required for the Fast of Esther.
If the 13th of Adar falls on Shabbat, the Fast is observed the preceding Thursday which is the eleventh of Adar. Be. cause of Purim, the Fast is not postponed to the following day, nor is it observed Erev Shabat: Since it is no longer observed in any event in its proper time, it was not fixed for Erev Shabat, in deference to the honor of Shabbat. (A Fast whose prescribed date can fall on Erev Shabat such as the 10th of Tevet, is neither postponed nor observed earlier, but it is observed on its fixed day).Tachanun is not said during minchah of the Fast of Esther.
On the 13th of Adar during minchah, it is customary to give three halves of the coin which is the basis of the local currency. The money is given to the poor to do with it as they wish. This contribution is made in memory of the half-shekel given by Israel when the Beit Hamikdash still stood; and whose forthcoming collection was announced on Rosh Chodesh Adar.
This memorial act is performed before the Reading of the Megilah, because all Israel gathers for the Megilah Reading in the synagogues. It is proper to give the half-shekel before minchah, since 'the diligent perform mitzvot earlier.' Those who live in 'open-cities' give the half-shekel before the Megilah Reading on the night of the 14th, whereas the inhabitants of Yerushalayim give the half-shekel before their Reading of the Megilah the night of the 15th.
In a place which has no coin that is designated a 'half' coin, it is customary for the gabaim to bring three halves of silver coins which are issued elsewhere, and to give these coins in exchange, to anyone who makes his contribution in the coins available to him. After performing performing the mitzvah, he returns the three 'halves' to the gabaim, so that others might also be able to observe the custom properly.
Those who seek to observe mitzvot with hidur (enhancement) give the half-shekel for each of the members of the household including minors, and in the case of an expectant mother, for the unborn child as well. Once a father has begun to give a half-shekel for a minor child, he is required to continue to do so each year.
The reason for the giving of three 'halves' is that the term trumah (contribution) is mentioned three times in the portion of Ki-Tisa, in the account of the mitzvah of the halfshekel. The established practice is to consieder the giving of the half-shekel as not freeing one from the mitzvah of giving charity to the poor, which is specifically prescribed for Purim.The 13th of Adar is also mentioned in the Talmud as the day on which vengeance was executed (during the time of the Hasmoneans) against a tyrant who oppressed the land of Yehudah cruelly and arrogantly blasphemed the city of God. The name of the tyrant was Nikanor and he fell by the hand of Yehudah, the son of Matityahu, on the 13th of Adar, which was hence celebrated as a festive day.
Sources: Orthodox Union