Question:

As I understand it, Esther called for a three-day fast before she went to see the king. The three days lasted until the first day of Passover—so they fasted that year instead of eating the Passover festive meal.

Now we are making a fast on the day before Purim. I see that the reason given is because they probably fasted on that day, since they were praying to be delivered from their enemies. But if so, why do we call it the “Fast of Esther”? Esther’s fast was the three-day one which included the first day of Passover. Obviously, we’re not going to do that every year. But why call this fast the Fast of Esther?

Answer:

Great question. Actually, the Rebbe, Rabbi Menachem Mendel Schneerson, of righteous memory, asked the same thing.

His explanation was that Esther was the only one who could have fasted on that day. The rest of the people might have felt they should be fasting, but were not permitted to, since they needed every ounce of strength to defend themselves against Haman’s venomous hordes.

Esther, sitting in the palace, was the only one who had nothing to fear, and so was permitted to fast.

Since she was the only one who fasted that first time around, we call the fast after her.

Sources
Based on the Rebbe’s talk on Purim 5730 (1970).
See also Likkutei Sichot, vol. 6, pp. 371–372.