In the first informal Halloween event, Halloween costumes where introduced with Version 1.7.7 in 2014. The costumes were composed of a Halloween color pack and a jack o'lantern accessory. With the release of Version 1.14, the original Halloween costume pack is now an unlockable item in the Shop for 20000. These are the only Halloween costumes that are not seasonal post-release.
The 2014 Halloween Event is a seasonal event that ran from October 26, 2014 - November 11th, 2014.
In order to unlock the Halloween costume pack in the shop, the player must play 20 games as a character dressed in the black color pack. Multiplayer games would count as two games for the purposes of this unlock.
The Halloween costume pack unlocks both the Jack o'Lantern accessory and the Halloween color pack for all characters. Before Version 1.14, the accessory and color pack counted as one item and could not be equipped separately.
As of Version 1.14, the costume pack is no longer seasonal and can be purchased in the shop at any time. The requirement to make it available for purchase is the same as it was during the event.