Last night I was planning on going to the gym with my boyfriend this morning, but I woke up and could barely move. My eyes were all puffy, it hurt to sit on the toilet to pee and I could barely put my hair in a pony tail my arms hurt so bad. Well that means its a rest day for me!
I don't plan my rest days ahead of time, I see how I'm feeling that day and decide from there. I might wake up thinking I'll take a day off from the gym but end up doing some of my best workouts later that night.
Some people need to have a set day every week, but for me this is difficult to do for many reasons. Now these reasons could be called "excuses" for not working out, but I know how my body functions and if I'm physically capable of getting in a workout for that day.
You know your body better then anyone, yes you need to push yourself but that is very different from knowing your breaking point. The more you work out the better you will be able to tell the difference between being lazy and needing a rest day.