The Wedding Night of River Song is the terrible disaster that the reapers from Father's Day were trying to prevent. It stars nobody, and wasn't so much written by Moffat as it was accidentally submitted and produced instead of an actual script.
River refuses to assassinate her luvvybum, and as a result the space time continuum kinda takes it in the ass. You know how time can be rewritten? Well, some bits can't; for example, good episodes. As a result of a paradox involving an extraordinarily long and loopy timeline and multiple people being present for events multiple times via time travel, time crashes and "nothing happened. And then it kept happening. Or, if you'd prefer, everything happened at once, and it won't ever stop."
The result of this cataclysmic time crash? A really fun cold open, followed by a lot of talky bits and weird stuff that all feels really crammed in. Probably could have stood to stretch this out to two episodes, but then we would have missed out on that season's beloved christmas special The Doctor, the Widow and the Wardrobe.
Also there's a bit at the end where they make the title of the show into a central theme, because meta fiction is super cool you guys.
Boy, this grand and epic finale sure didn't have any lasting effects. Literally every major development is forgotten or undone within a few episodes, with the exception of the Brig's death and the revelation of what the Question was. Not that it mattered.