I just saw this essay of Rav Yonasan Sacks on ta’anis esther and megillah and thought it fit well with the idea of the Rav in the previous post. The gemara derives from derashos that megillah can be read by villagers on 11 and 12 Adar, and notes that it obviously can be read on 13 Adar because that day is a “zman kehilah”. The Rosh interprets zman kehilah as a reference to gathering for the fast. Why, asks R’ Chaim Turtzin, should this be a mechayeiv of kerias megillah? What does the fast have to do with the celebration of the miracle of being saved?
R’ Sacks quotes the Rambam in the conclusion of his intro to the Yad that the theme of the megillah emphasizes Hashem’s bringing redemption from danger in response to our prayers. Rav Sacks concludes that, “On Purim, we do not merely celebrate the miracles themselves, but rather, the metamorphosis from disaster to tranquility. The contrast is what is critical.” That being the case, ta’anis Esther, whose fasting marks the potential tragedy Haman planned, is an essential component of the pirsumei nisa of Purim itself.
Perhaps this is the difference between the keriah at night, which the Rav saw as preparatory to the actual mitzvas hayom of Purim, and the kerih during the day, which Tosfos’ holds is the primary reading. During the reading of the night, coming immediately at the close of the ta’anis, we are captivated by the danger of the story, as the tragic plans of Haman unfold before our eyes. This reading stands in contrast to the reading during the day, when we already know how the story will end, and focus our attention more on the happy conclusion to be celebrated through seudah and mishloach manos.