[this is based on a post from 2 years ago and will be published in Peiros Ha'aretz.]
This coming Thursday, the 13th of Adar, is Ta'anis Esther, a puzzling fast day. According to many opinions, this fast commemorates the fast the Jews fought during the war against their enemies on the 13th of Adar. According to others, it is in memory of the 3 days the Jews fasted before Esther went to see Achashverosh. That occurred during Pesach time, so it seems strange to mark this fast on the wrong date. But either way, why is it necessary to fast, no tragedy happened on Ta'anis Esther! Also, according to some opinions, it shold be forbidden to fast on the day before a holiday! What was the great need to fast before Purim?
Purim is an unusually jubilant day in the Jewish Calendar. There are the mitzvoth of mishloach manot and the festive seudah, and there's even a mitzvah to get drunk! This is besides all the customs that have become common, such as wearing costumes.
But matters were very different for the Jews in the times of Haman. A royal decree had been passed, calling for the annihilation of all the Jews. People did not know what would happen. It was scary times.
In the end, the Jews were miraculously saved, but because of an important reason: They did Teshuva. Haman had caused the Jews to wake up, and they fasted and repented. G-d then caused the miraculous turnaround of Purim.
On Purim, when we celebrate with such ecstasy, there's a risk we might become too light-hearted. There are still those who seek to destroy us, and it is vital we don't miss the message. Therefore, on the day before Purim, we recall the troubles and worries the Jews went through, and more importantly, we remember the reason they were saved, and we do teshuva ourselves. Only then can we celebrate a meaningful Purim.
