From November 2nd to January 3rds.
Microsoft has something big planned for this holiday period, and that plan is to sell an off the chart amount of Xbox Ones. In the US only, the console will be $50 off starting November 2nd and going through January 3rd. This offer applies to all console bundles at participating retailers like Amazon. (Other retailers specified by Microsoft include Best Buy, GameStop, Microsoft Stores, Target, Toys ‘R Us, and Walmart.)
From Microsoft, "Fans will have a broad portfolio of amazing games on Xbox One this holiday. Only on Xbox One can you play exclusive games like Halo: The Master Chief Collection, Sunset Overdrive and Forza Horizon 2, plus the newest blockbusters such as Assassin’s Creed: Unity, Call of Duty: Advanced Warfare, Destiny, Dragon Age: Inquisition, FIFA 15, Madden NFL 15, Grand Theft Auto V, Minecraft and dozens more this holiday."
This means that yes, the new 'Assassin's Creed' bundles, which include 'Assassin's Creed Unity' and 'Assassin's Creed IV Black Flag', are $50 off, making them $349 (without Kinect) and $449 (with Kinect).
And then there's the 'Sunset Overdrive' bundle, which includes the white Xbox One console and the game for $349.
Meanwhile, the 'Call of Duty: Advanced Warfare' bundle, which is the only custom art model, and the only one with a 1TB hard drive, includes the game and console for $449.
And finally, just the regular Xbox One is $349, which means those other bundles are a much better value, and will be in short supply first.
Source: Major Nelson