My koi is an offline game, but during events the developer releases a special version of the game that allows you to create special koi that otherwise would be unobtainable.
This guide tells you how to download that special version irregardless of the date of the year. You no longer need to wait to get a special koi.

Press Windows key + R then type in steam://open/console to open Steam console.
Then type download_depot 1114130 1114131 7971263362744375580
to download the Halloween update.
Now open the path that the steam console mentioned
"C:\Program Files (x86)\Steam\steamapps\content\app_1114130\depot_1114131"
and the main game folder, in explorer.
And copy all of the files in the depot folder to the main game folder.
Open the game and claim your skull koi.

This video explains how to get the numbers used in the retrieval of the Halloween update of the game. The ones issued in the steam console with the download_depot command.