Soundwave on 25 December 2013
Makes sense, even if Nintendo "only" sold say 350k in December, the majority of systems purchased in December are meant as gifts for this day, so you will have an unusually large number of systems trying to access Nintendo's servers on one day probably well over 10-20x more than a normal day.
You probably also could have existing Wii U owners who recieved money today as part of their gifts and are trying to log on so they can download a game or something.
