Blizzard has posted a brief notice on the World of Warcraft forums to let players know that the Nighthold raid will be open for Normal and Heroic play on January 17, 2017 for North America and on January 18, 2017 for Europe. The Mythic and Raid Finder versions will be released in the weeks following that, though more precise dates are not yet available.
Devs will also be releasing a 'tuning update' for the Trials of Valor raid, specifically Normal and Heroic versions, to give players time to progress through prior to the release of Nighthold.
Check out the World of Warcraft forum for more information.
