Both the eShops for the Wii U and 3DS will be undergoing extended maintenance on Thursday afternoon.
According to Nintendo's network maintenance page, starting at 1:00 p.m. PT and until 5:00 p.m. PT, Wii U and 3DS eShops will be undergoing maintenance. During that time, services may not disappear entirely, but Nintendo warns, "During the extended maintenance, parts of the affected services may be intermittently unavailable."
Along with the eShop maintenance, from Monday, March 10 starting at 5:00 p.m. PT and until Tuesday, March 11, 7:00 a.m. PT, online ranking and gameplay for Pokémon X and Pokémon Y will be affected by maintenance.
The hope is that Nintendo is prepping its eShop servers for something exciting, but in reality, it's probably just some normal maintenance to keep everything running smoothly. If you've been putting off purchasing anything from the store, make sure to do it before Thursday.