It will affect most online services, and will mean that the 3DS and Wii U Shop, Wii Shop Channel and DSi Shop will all be inaccessible. This will last from 11pm on Monday the 28th January 2013 to 5pm on Tuesday the 29th (Pacific Time Zone).
Here’s the list of services affected by this throughout the entire duration:
Here’s the list of services that will be inaccessible from 5-6pm on Monday the 28th:
And here’s the list that will be unavailable from 12pm to 5pm at that time:
Sorry for the inconvenience this might cause, Nintendo are apparently just doing routine maintenance or something and need to take their shops/online servers offline for a few hours as a result.