The Xbox One will receive a mandatory update at 8PM ET. The update is automatically applied to consoles in the "Instant On" low-power state, which also allows the system to be turned on by voice. Those who power the console off completely will be required to download the update after Thursday, December 12 at 7PM ET.
The update addresses SmartGlass standby issues, multiplayer problems when rejoining games, notification glitches, dashboard performance and the console's wireless networking driver. It also updates the TV, system update and content update services.
For consoles in "Instant On" mode, the update will automatically begin downloading in the background once the console is off. Then the box will power down completely and users will have to turn it back on manually for the update to be applied. See the full details from Major Nelson