Yesterday my daughter told me her Morah said she asked a good question for which her Morah had no answer. I am always more convinced my kids are actually learning something when they are asking good questions rather than functioning like parrots of all the good information they learn (do parrots, or children trained to be parrots, actually learn anything?) But I digress...
My daughter Rachel asked: how is it that Esther fasted instead of fulfilling the mitzvos of Pesach night (which require eating)?
The simple answer probably is that while you cannot take a shevua which would cause you to violate a mitzva, the Mishna in Nedarim tells us that a neder is chal even on a dvar mitzva. So you can take a neder not to eat and fulfill the mitzvos halayla of pesach.
On a less simple level, the Radomsker asks: how is it that Esther knew that the zechus of ta'anis would overcome the gezeirah? Why didn't she think that the zechus of the mitzvos of leil haseder would do the trick, and by enacting a ta'anis she was sacrificing the necessary zechoyus? Explains the Radomsker: every year, on lein haseder, the malachim sit in shamayim waiting to carry the zechuyos of our mitzvos up to the kisei hakavod. On the year of Purim, as every year, the malachim stood waiting... and waiting... and waiting, and nothing happened. When the malachim realized there were no mitzvos, there was a tremendous tumult in shamayim to discover what was wrong and what caused there to be no Pesach that year. If all the malachei hasahreis were commotion for this one lost pesach, kal v'chomer the uproar if chas v'shalom Haman's gezeirah was brought to fruition. This was the pleading and limud zechus on behalf of klal yisrael that turned the tide.