On my first Camino, I planned a rest day every week, as my training suggested that my body would need that. After getting out on Camino and actually taking my first rest day, it felt very weird NOT to spend the day walking. I've never taken one since. What many veterans do is take "Short" days of 10 Km or so. I usually plan these coming out of a larger town, allowing for a later night out at a real restaurant, and a leisurely multi coffee start to the day. For me, 10 or so IS a rest day, but lets me stay mentally in the Camino zone. Any type of injury will, of course, torpedo your planning. If you are hurt, hole up and recover.