The Halloween Party is an easter egg that occurs on 31st October each year to celebrate Halloween.
During Halloween, random NPCs will wear a pumpkin on their heads, and in some cases, Jack o' Lanterns . This Easter egg has been in place since Public Beta 9.
In addition to wearing pumpkins on their heads, most NPCs have a chance to say "Happy Halloween, Player!" when spoken to. This feature was added in Public Beta 26 alongside similar speechbanks for April Fools' Day, Christmas and New Years Day.