אקדומי פורענותא לא מקדמינן?

I hope you'll all forgive the Hebrew title, but it's kind of hard to translate meaningfully. Let me explain.

In Jewish law, when certain fast days fall on Saturday, we put them off to Sunday. The exception to this is the Fast of Esther, which is advanced to Thursday. The reason? Because the other fast days that are put off relate to tragedies and we don't advance the observance of tragedies.