f you can't get on to the Nintendo eShop to download games or updates for your game console, don't panic. Nintendo is taking the store offline to fix connectivity problems that cropped up over the last few days. The eShop will be unavailable from December 27 at 4 p.m. ET/1 p.m. PT to December 28 at 4 a.m. ET/1 a.m. PT.
The issues arose on Christmas Day as people with a new Wii U or 3DS systems were setting up their consoles, creating new Nintendo Network IDs, downloading updates, and purchasing games. The overwhelming volume of traffic to the store prevented others from connecting to the eShop, Nintendo said.
Nintendo posted a notice on their Facebook page to give customers a heads-up about the shutdown:

We sincerely apologize for the connection problems in the Nintendo eShop. We understand this is taking longer than expected, but we can assure you that providing a solution is our top priority. We are determined to make sure everyone can enjoy all that Wii U and Nintendo 3DS have to offer. In an effort to manage the high volume of traffic and ultimately improve your experience, we are temporarily taking the Nintendo eShop services offline between the hours of 4 p.m. - 4 a.m. ET. Thank you so much for your continued patience. We will keep you updated.

An earlier tweet from Nintendo's UK Twitter account noted that the UK eShop would go down, but it appears this is a worldwide issue affecting the online store in every market.
Also because of the connection problems, Nintendo is postponing the launch of two new Pokémon games, Pokémon Poke and Bank. They were slated for release on December 27 and there's no word yet on when they'll be available.