On my first CF, I planned rest days in Burgos and Leon. I then only stayed one night in Burgos, and took the day in Leon. On subsequent pilgrimages, I have allocated a rest day for every 10 days walking, and a 'sick day'. This is really only to make sure that I am not cutting things too fine in my planning, and doesn't necessarily mean that I will take them at any particular place.
As an aside, when I walked the Gudbrandsdalen, one of the St Olav's Ways in Norway, there was a group walking who took a rest day every Sunday and attended the local church service. Their walk was coordinated by the Norse Pilgrim Office, and the Sunday service became the Pilgrim Mass for the parish. I was fortunate enough to cross paths with them, and stay in the same place on the Saturday night. On the Sunday, I attended the church service with the other pilgrims in the group. It was followed by a wonderful 'morning tea' - a feast of cakes and scones put on by the parishioners for the pilgrims. It is high on the list of wonderful moments of generosity that I have experienced when on pilgrimage.