The Fourth of Christmasween is an event that occurred in a July 2015 stream.
The Stream Edit
When Mike got into a Terraria server it was going well at first, barring some lag here and there. Bern was denied owning Reisen several times by Mike. So she decided to punish the filthy human mortals who denied her of owning the moon rabbit.
Come midnight in Terraria, all hell broke loose. Bern began to summon bosses left and right, mostly destroyers and the likes of Cthulhu Eyes and Skeletrons. Then game lag caused both a blood and frost moon to happen at once, bringing forth the two very opposite waves of creatures to appear simultaneously. On one side there was vampires, zombies, frankensteins, Swamp Monsters, Werewolves, Grim Reapers, and possessed souls running amok. On the other side was snowmen, Christmas elves, Christmas tree monsters, gingerbread men, reindeer, zombie elves, and Christmas ornaments running amok. And in the middle were all the bosses. After a while it turned into a bullet hell danmaku of halloween, christmas, and boss monsters. Death and anarchy reigned by these three destructive forces. After the events, Mike did a mock eulogy of everyone who died and got in the middle of the action.
Later on, however, Christmasween came back, much to everyones' (but mostly Mike's) displeasure. It got so convoluted that the server crashed. Thus, the wrath of the Fourth of Christmasween ended. Everyone gave Bern flack that day.
It as given its name as an Amalgam of The Fourth of July, Halloween, and Christmas. This is due to the events and the fact that the even happened mostly on July 4th.