We’ve summed it up here, or, more accurately, we’ve stolen a nice summary from Wikipedia:
FOX will air Season 7 beginning with a two-night, four-hour premiere over two consecutive nights.
The first two hours will air on Sunday, January 11, 2009, and the third and fourth hours on Monday, January 12, 2009. From thereafter, 24 will air the remainder of the season on the following Mondays, without interruption.
Sky1 will broadcast a two-hour premiere special on Monday 12 January, another two hours the following Monday, and a single episode on every subsequent Monday, meaning that the UK will be a week behind the US, due to it being broadcast on the same night in both countries.