Today, the 13th of Adar, was a critical day in the times of Mordechai and Esther. This was the day selected by Haman to commit genocide against the Jewish people. Miraculously, the Jews gained the right to defend themselves against their enemies. The Midrash recounts that the Jews actually fasted as a nation on that day, in supplication to Hashem for Heavenly assistance. Moshe Rabenu also fasted while presiding over the battle with Amalek in prayer.
Each year we commemorate the miraculous events of this day by fasting. While the fast is known as "Taanit Esther-The Fast of Esther," it is not connected to Queen Esther’s fasting, but, rather, to the Jews fasting during battle. When the 13th of Adar falls on Shabbat, the fast is pushed back to Thursday. The fast begins at dawn and ends at nightfall. During Mincha, the special Torah portion of VaYachal is read, and those who are fasting wear Talit and Tefilin.
There is a question as to why it is permitted to fast so close to Purim. The holiday of Purim is recorded in the early work, Megilat Taanit, as one of the happy occasions on which it is prohibited to fast. There is a general principle that it is prohibited to fast on the day before and after any holiday recorded in Megilat Taanit. Even though later, the list of holidays in Megilat Taanit was nullified, Purim retained its status. The question then arises, why is it permitted to fast on Taanit Esther, the day before Purim? The answer is that when Megilat Taanit was nullified and Purim was readopted, the Hachamim did not re-adopt the special status of the day before and after Purim.
The consensus of the Poskim is that Taanit Esther has the status of Minhag-custom. This is the opinion of the Rambam and Rashi. Hacham Ovadia understands the opinion of Rabenu Tam this way as well. Nevertheless, "Al Tifrosh Min Hasibur-Do not separate yourself from the community." Everyone should make an effort to fast. In fact, the Ran holds that this fast has the status of Divreh Kabbalah-an institution of the prophets, as it is referred to in the Megila.
There are a number of cases in which a person is exempt from fasting on Taanit Esther:
1. Someone suffering from an illness, even not life-threatening, Chas V’Shalom.
2. A pregnant woman after the first trimester. Before the end of the third month, Hacham Ovadia rules that she must fast, whereas Hacham Ben Sion ruled that she is exempt as soon as hears the good news that she is pregnant.
3. An actively nursing mother is exempt for up to 24 months from birth. Hacham Ovadia ruled that if she stopped nursing, she retains the exemption for the entire 24 month period, if she feels weak, since the body is still recovering. In such a case, Hacham Ben Sion allows an exemption for only 30 days from when she stopped nursing. If a woman is not nursing, she is exempt for 30 days from the birth. This applies also to miscarriages.
4. Ba’ale Brit Avraham- The father of the baby, the Mohel and the Sandak are exempt from fasting on the day of the Brit. Hacham Ovadia rules that it is their Yom Tob, and they should eat.
5. A Hatan and Kallah during the week of Sheva Berachot are exempt from fasting.
In all of the above cases, one who is exempt does not have to make up the fast after Purim. On the other hand, if someone is not ill, but has a headache, while he is exempt from fasting, he must make up the fast after Purim.