It doesn't matter if you're sad or hung-over or lazy or tired - a workout will get your endorphins pumping, and you'll feel like a new person almost instantly.
Once your body is in workout-mode, a few days off won't hurt. Muscle memory is magical. If you work out consistently, you can afford to miss a few sessions and your body will gladly pick up where you left off.
Look, maybe I'm just not good at multi-tasking and am, therefore, jealous of those of you who can get in a workout while yammering on your cell phone, but for the love of all that is good and pure, shut your yap!
I used to hate working out - until I learned how to do it properly.