It just makes sense to hold events during the times when most people can play. Judging by perusing the forums periodically, EM events are held at all different days at all different times. Centrally-run events have on occasion been held during off-peak times, with the result being that people complain they cannot be gotten to. Contrary to what someone implied in this very thread, nothing prevents one from coming to an event save for one's own schedule and play preferences. EM events used to be held at odd times more often, during the first run of the program, with the result being that people complained they couldn't be gotten to. So, yeah.
-Galen's player