From Thursday, February 13, 9pm Pacific / 12am Eastern / 5am UK / 6am CET to Friday, February 14, 2014 12am Pacific / 3am Eastern / 8am UK / 9am CET
Affected Services:
Wii U
Online Play, Rankings, etc.
Nintendo 3DS
Online Play, Rankings, etc.
Also worth noting is that there'll be extended maintenance to the online services of Pokémon X & Y at the weekend / early next week, details below.
http://www.nintendolife.com/news/2014/02/extended_nintendo_network_maintenance_scheduled_for_later_today
From Sunday, February 16, 2014 5pm Pacific / 8pm Eastern / 1am (17th February) UK / 2am CET - Tuesday, February 18, 2014 4am Pacific / 7am Eastern / 12pm UK / 1pm / CET
Affected Services:
Nintendo 3DS
Online Play, Rankings, etc. for Pokémon X, Pokémon Y
