I've read that the reduction of light hitting a deer's retina is what triggers the rut. Day length and cloud cover would then be the cause of rutting. I probably read it on a hunting bulletin board, but someone I have hunted with got the same story from a game warden.
Most of the rut activity I've seen has been in the late season but some has been in late October. These two pictures are of the same blacktail killed last year on October 26th. Judging by the neck I'd say he was into rut.