Introduced with Version 1.8.4 in 2014, the first Christmas event added Santa Hat accessories for characters to equip. Aru was made available for all players to use, and all CPUs wore Santa Hats for the duration of the event.
The 2014 Christmas Event is a seasonal event that ran from December 24, 2014 - January 12th, 2015.
The Santa Hat pack unlocks the accessory for all characters.
As of the conclusion of the event in Version 1.8.6, the accessories are not seasonal post-release. The method to unlock the pack remains the same, except the pack must now also be purchased in the Shop for 20000 after fulfilling the unlock condition.