For Wii U and 3DS
Nintendo is planning extended maintenance this week, and it will take both the Wii U and 3DS eShops offline for a bit. It starts on Thursday, March 13th at 4PM EST, and ends at 8PM EST.
Pokemon X and Y also encountered maintenance from March 10th (8PM EST) until March 11th (10AM EST), at which point all online services were be down. That time is over now, but there could be lingering issues so sit tight. So yeah, if you're planning on buying something or playing Pokemon, plan accordingly!