This event repeats every year until 01/21/99
WARNING: Season 2, Episode 14 SPOILERS!
In today's episode, Rainbow Dash called Derpy by name, rendering her previously fanon name official. Derpy then went on to have a full speaking role for a good minute or so in which she destroyed part of Ponyville's town hall, much to Rainbow Dash's frustration.
This event is significant because it proves, once and for all, that the show's creators are well aware of their fandom and listen to their viewers. Derpy originally began as a simple animation error in the show's pilot episode, but became a hit character among the fans.