The check on the calendar is your mark on that particular day telling you that you did something for your body. Generally, when I refer to making a mark on the calendar it means that for some reason you were not up to hitting it hard. Instead, you worked out at a lower intensity but still did something for your physical fitness.

It has been my experience that simply beginning a workout, even though you aren't feeling up to it, will in the end turn into a good if not a great workout. If not, then at least you did something, i.e. the check on the calendar.