Why would it be on now and clicking wildly on wednesday?
Because it's bad.
I'm a firm believer that there is a little troll that resides in all solenoids. When he gets tired and wore out from years of water, grime, and vibrating beatings he likes to play tricks... I've had many o' run in with this little bastard...
If you connect the ground and it continuously cranks over it's usually the solenoid being stuck. Just like when you jump the 2 terminals with a screw driver...
There is a possibility that the start button is stuck too but usually these stop working from corrosion instead of getting stuck on...