I am not saying slack off but I do think its important to give ourselves one GRACE day where we don't have to follow rules or be perfect. If we know we can give ourselves one day a week like this than it is less likely that we will slack off for longer periods of time. When you do have a hard day... give yourself grace to forget the calories and lack of exercise. Every day is new with no mistakes in it so live it like yesterday didn't exist!
See my blog for more info on stress/accomplishment journals.