The Party Hat is an accessory.
Available Party Hat Accessories
Prismatic Aspect: None.
This accessory was first made available during the 1st Anniversary of Spiral Knights, and have been every anniversary since. Some hats were available on the supply depot around the New Year of 2013-2014 (so this accessory is not limited to just the Caketastrophe).
A Party Hat is part of a group of celebratory accessories, which include Party Hats, Birthday Candles, Grand Toppers, and Party Blowouts, as well as Sprinkle Auras.
There are several different patterns on these hats, such as polka dots or stripes. Wolver Tails also have this "different patterns for different color styles" art.