The big new update that Microsoft announced last week is reportedly being targeted for a 25th of November release date. Though what exactly "targeting" means is anyone's guess. Last week's announcement told us a lot about what Microsoft is planning for the update, including a big new push into providing television content, but until now we didn't know when the update would be coming.
The use of the vague word "targeting" likely means that they'll do their best to launch on that day, but aren't ready to make any promises. Either that or they're worried that having 1/3 of the human race playing Modern Warfare 3, Battlefield 3, or FIFA 12 will make XBL inaccessible (not really.)
We're guessing that even if the 25th isn't a hard deadline, we'll still be seeing it very close to that date. You can check out the new update's introduction video over at Eurogamer.