Come March 13, both the Wii U and 3DS will undergo a scheduled maintenance day that will affect the eShop of the aforementioned systems.
The following information comes from NintendoLife, who did a great job of laying out what time system owners can anticipate the maintenance to go down.
From Thursday, March 13, 2014 1pm Pacific / 4pm Eastern / 8pm UK / 9pm CET to Thursday, March 13, 2014 5pm Pacific / 8pm Eastern / midnight UK (14th March) / 1am CET.
NintendoLife also went on to explain that the online play for Pokemon X and Pokemon Y will be unavailable for about 14 hours starting on March 10.
This may also have an effect on the Pokemon Bank application, which has its free trial period coming to an end on March 14. For those that haven't downloaded the app yet it would seem that now is as good a time as ever.