The space shuttle is rolled out to the pad about a month early mostly empty, no payload and just the SRBs are fueled. once it is on the pad it will normally stay there until launch, most of the time the Rotating Service Structure will be covering the shuttle form weather. at the pad the load the payload on through the RSS and then 12h before launch they start loading it up with liquid fuel.
What I am trying to say is the shuttle will stay out on the pad once it is put out there, it normaly takes 8 hours to park it out ther, and then 4 hours to pull the Crawler back once the Moblie Launch Pad is locked in place, so in all to move the shuttle to a launch pad will require atleast 12 hours. It makes leaving the shuttle out on the pad much more likely.