Off days are simply not a thing. You can either be like physically sick and in that case you take the day off but there are no off days. If you are good at something and you can do things in a single threaded manner with focus and you have one to two things which are major items that you want to check off, every single day will be a good day. Now you might not make as much progress or some days you might make a lot of progress, but the days in which you do not have progress are not off days.