On a week long trip, it is rare to take a rest day - though when I've taken them it is because some major event like aftermath of a hurricane coming through with lots of rain. On trips longer than a week, I've taken more of them. Typically for one of three reasons (a) weather (b) mechanical problem (c) stop through a tourist area and see attractions. They aren't necessarily planned though I'll anticipate some in tourist areas. I've also taken occasional shorter days. Those have been more typically weather related. For example, after cycling for several hours against the wind - decide it would be at least as much fun to rest in the afternoon. Alternately, have had a day with high wind and tornado warnings so seemed wiser to stop that day.