Ah ... I just let someone else make the decision for me ... when I go to the gym, I dip my hand in the lap pool on my way through to the locker room, if it feels too cold, that means it's a good day to do something else.

WTH? ... can't the $4 billion dollar corporation that owns my gym pay its gas bill?

All joking aside, there are a lot of different training regimens and schedules out there ... Training Peaks is a good iPhone (I assume Android as well) app that gives you a workout schedule day by day and a lot of good data tracking capability if you want it.