Emergency Nintendo Network maintenance will be going underway today starting at 3pm Eastern time and will last through tomorrow at 8pm Eastern time. All current-gen Nintendo systems are being affected by the maintenance, including the Wii, Wii U, DS and 3DS eShops.
A full maintenance schedule can be found here on Nintendo’s site. Matchmaking for certain games will be unavailable, including Warriors Orochi 3 Hyper and Pokemon Black & White and Pokemon Black & White 2. Nintendo hasn’t given an official reason for the maintenance, but considering Iwata’s assurance that the company is working to fix issues with lag, connectivity, and the speed of updates, it probably makes sense that maintenance is a top priority.

Affected Services


Thanks to G-Money for the heads up on this topic
Source
Source