Today is the public fast day known as the “Fast of Esther” and it is customary among the entire Jewish nation to fast on this day.
In the days of Mordechai and Esther, the Jewish people gathered together on the 13th of Adar to defend themselves against their hateful enemies and they were in need of great Heavenly mercy in order to be saved from certain annihilation Thus, they stood in prayer, supplication, and fasting on that day just as their forefather, Moshe Rabbeinu, did on the day that Israel battled Amalek when he stood in prayer and fasting, and the Jewish nation was victorious. Hashem, G-d of our forefathers, heard their cries and graciously accepted their fasting and repentance, and on the same day the enemies of the Jewish people thought to gain control over the Jewish people, a switch occurred and the Jewish nation gained control over their enemies and the Jews killed 75,000 of their enemies besides for the enemies killed in the Persian capital of Shushan. Not one Jew fell, for this war was not waged with might and power; rather, it was fought with the guidance of the spirit of Hashem. Therefore, the Jewish people customarily fast on this day every year to commemorate the miracle that was performed for them. This fast is called “The Fast of Esther.”
The Meiri writes in his Sefer Bet Ha’Bechira (Ta’anit 18a) that some say that the Fast of Esther is a fast of joy and is not included in the group of the other public fast days which mark sorrowful events. Others disagree.
Pregnant and nursing women are exempt from the Fast of Esther for if they are even exempt from the four fasts stated explicitly in the verse in Zecharia (8, 19) with the exclusion of Tisha Be’av, they are certainly exempt from the Fast of Esther. The Rama (Rabbeinu Moshe Isserlish) also writes in his notation on the Shulchan Aruch (Chapter 686, Section 2) that pregnant and nursing women are exempt from the Fast of Esther. Maran Ha’Shulchan Aruch did not need to write this law explicitly for it is self-understood from the rest of the fasts; however, the Rama did need to write this explicitly according to the Ashkenazi custom that pregnant and nursing women do fast on the rest of the Four Fasts unless they are suffering greatly. Therefore, the Rama felt the need to write that at least on the Fast of Esther they are exempt from fasting.
The halachic definition of a “pregnant woman” regarding being exempt from this fast is a woman who has already passed the first three months of pregnancy. However, if a pregnant woman suffers from nausea and morning sickness, she can be lenient and not fast even before three months of pregnancy have passed, especially if forty days have elapsed from the onset of the pregnancy.
Regarding the exemption of a nursing woman, even if she is not actually nursing, as long as she is within the twenty-four-month period since giving birth, if she feels weak, she may act leniently and she is not obligated to fast. However, if she feels healthy and strong, it is preferable for her to act stringently and fast.