The price drop, which is described as a "promotion" in the announcement, doesn't come with an end date. If this sounds confusingly familiar, that's because Microsoft instituted an identical price drop over the holiday season, which ended as of January 3rd. If you bought an Xbox One at the $399 price in the roughly two weeks between then and the upcoming drop, I bet you're really, really happy right now.
Most of the announcement and accompanying video involves touting of sales numbers--noting that the system was the best-selling console over the months of November and December in the U.S. according to NPD--and some brief overviews of things on the horizon, like screenshot functionality and new apps for the system. You can read/watch the entire thing here.