How do you get mustards costume in AQW?

To obtain Mustard's costume in AdventureQuest Worlds (AQW), you need to complete the quest related to the Mustard character during the appropriate event, typically found in special seasonal events. Look for quests that involve collecting specific items or defeating certain enemies. Once you complete the requirements, you can claim the costume as a reward. Always check the AQW forums or official announcements for the most current information about available items and events.