Josh Marshal suggests that W will fill the AG's position with a recess appointment. Maybe that explains why they let Gonzalez's resignation out of the bag on Monday morning, instead of late on Friday afternoon, on the eve of the Labor Day weekend. But I suspect that even Bush realizes that using a recess appointment to get in someone Congress would not approve would only prolong the uproar over the way that he has treated the Justice Dept. and the law generally.
A clue to his intentions may be found in his statement on the resignation this morning (if you can bear to watch his self-pitying distortion (to be kind) of reality). At the end, he says that he has asked Paul Clement, the Solicitor-General, to be acting AG until a new nominee is confirmed by the Senate. You can see the statement here:
My theory about the timing is that too many people knew what was going on: the story would have broken long before Friday and they'd have looked even worse if they tried to hold it that long.