I think nobody is on for tonight, so why not? I’ll give it a shot (By the way, any information leading to the whereabouts of Moderator 72 would be appreciated)
What is Ta’anis Esther about? Why do we fast? Since we could have been killed (but were saved) we fast? NO, WE CELEBRATE! (the same question also applies to ta’anis bechoros, but we won’t be discussing that)
Also, whenever the calendar conflicts with a ta’anis, the ta’anis gets pushed off. The lone exception being ta’anis esther. I remember hearing from Rebbeim that this was since it was a “happy fast” (I thought it was so Purim would stay the same).
So, what gives? I saw the following answer (sorry, forgot who). The reason we fast is a remembrance for the 3 day fast that they did during the megilla. It is a day to remind us of the power of fasting, and the power of our tfillos. This is why we fast, even though there was no actual harm done to us. This is also why we move it up, as it is unlike any other fast.