There's normally an hour or two of maintenance each week on the Nintendo Network, often affecting Wii U and 3DS, though there's a slightly longer stretch coming shortly that'll affect 'online play, rankings etc' on the portable.
In total there'll be five hours of downtime; the eShop isn't mentioned specifically, but disruption could kick in with various online services; it's happening today (20th December) or tomorrow (21st December) depending on your timezone.
Online Play, Rankings, etc.
Some network services may become unavailable.
Date and time
From Tuesday, December 20, 2016 4pm Pacific / 7pm Eastern / midnight UK (21/12) / 1am CET / 10am AEST - Tuesday, December 20, 2016 9pm Pacific / midnight Eastern / 5am UK (21/12) / 6am CET / 3pm AEST.
Occasionally extended maintenance acts as a precursor to a system update or surprise treat, but in the minority of cases they pass with little incident. In any case, if you're plotting some online co-op in Metroid Prime: Federation Force, as we know everyone is, then beware the downtime.
Ok, so maybe it won't affect many people...
Thanks to all that sent this in.