My weekends are busy, so I tell folks that Monday is my real weekend - it's my day off since I work Saturdays and Sundays and I'm generally tired from the start.
So I make Mondays my heaviest day at the gym. Sound nuts I know, but part of my thinking is that if I don't get up and get going, I may well just drag around the whole day and waste it.
I left for the gym before 6.30 this morning, ran 7.5 miles and then did 2.5 miles on the eliptical, for a round 10 miles before breakfast.
Sure I'm shot now, but I sleep late on Tuesdays - no gym tomorrow, but a refreshed start to the work week!