I would like to say I have just the answer you need, but I have the same problem, especially Sunday night when the weekend is over. Then Monday I am extremely motivated again. I think it is because by Friday I haven't accomplished everything on my list and then I give up and give in to temptations.
Having said that, normally I don't want to go out on Saturday. I bum around the house in my pajamas or shorts. This last Saturday, I got up and mowed the lawn (forced myself to) and then took my daughters to a a downtown shopping area where we went to antique shops and just walked around. It was a lot of fun (once I got there). So I guess, even if you have to force yourself, or "pretend" to have fun while exercising or when you are feeling a bit blue will get you into a routine and once something is a routine it gets easier. That's my theory anyway