Tomorrow is the Fast of Esther. The fast normally falls on the thirteenth of Adar (the day before Purim), but is moved to the previous Thursday when the thirteenth falls on Shabbat. (Don't be confused by the fact that this post is dated "Thursday, March 4th." The fast begins at sunrise.)
Those who enjoy making trouble should note that it is actually forbidden to fast on the thirteenth of Adar according to a 1st-2nd century Aramaic work called Megillat Taanit. The thirteenth of Adar, according to this book, is the "Day of Nicanor," a holiday commemorating the victory of the Hasmoneans over an obscure Greek official. If you read Aramaic, you can learn all there is to know about the "Day of Nicanor" right here.
(FYI, a critical edition of Megillat Taanit was recently published. I haven't had a chance to look at it closely, but I'm told that it's a very fine work.)