Nintendo announced back in February that Nintendo WiFi connection services will be ending on May 20. That day is tomorrow, with several Wii and DS games being affected. If you still enjoy playing Wii games through Wii mode on your Wii U, this will affect you. The full list of games that are affected can be found here.
The Wii U and the Nintendo 3DS aren’t affected by the closure, but if you still have Pokemon from Black/White that you want to import into Pokemon X/Y, you’ll need to do so before tomorrow. This is just one example of the games being affected.