I have the habit of waking up before the alarm clock goes off just to make sure if I didn't set it I won't over-sleep. I've gotten the call from work (not from my current job, yet), and it can be pretty embarrassing, and not in a good way. You're in trouble, you're not getting paid for the time you're missing, and that's how it goes.
Saturdays I wake up at the same time as I do during the rest of the week, but I remind myself that it's Saturday and I don't have to get up. I usually don't set my alarm clock on Friday night, but last night, due to force of habit, I set my alarm clock, which didn't exactly thrill me when I had already returned to dreamland.
Mondays are kinda like that, too. Sorta like weekend jet-lag. I believe this condition can be commonly found in children. Parents can't get them up and out to school during the week, but once Saturday hits, they're up watching Saturday Morning Cartoons at some un-godly hour of the morning.
It's a puzzlement.