Originally scheduled for release on June 24th, the studio believes that the film is strong enough that it can hold its own at the box office through the holiday season.
No doubt that it's also serves the potential franchise better to move it out of the way of the summer sweepstakes and avoiding such films as Green Lantern, X-Men: First Class, Cowboys and Aliens, Harry Potter and the Deathly Hallows, Part 2, etc. By moving it, Caesar has a better chance of launching a series of films.