All wild creatures* on current Official Servers will spawn as Eerie during the event. Additionally, players will find that the days are shorter during this time of year.
On Unofficial Servers, server owners will find the option to turn on Wild Eerie Spawns through their web interface during the event. Please note that this setting will be forced back off at the end of the event, after daily maintenance.
*Due to technical issues, the Unicorn and Electrophorus will not spawn as Eerie.