If you plan to cheat you plan to fail.
If a whole day is dedicated to cheating, then you are putting yourself on a diet and giving yourself a food reward. Being primal is a change in attitude and expectations. It can be tough at first because those grains and especially wheat can be very tempting and addictive and many people experience withdrawal symptoms. As many here have reported as your taste-buds grow up you will find you will not like the taste of processed foods (perhaps even the smile).
Having said that the 80/20 rule is there for a reason. If you are out with friends enjoy. Don't binge on crap, eat the best you can but enjoy the night.
Eating primal is not a diet, it is a way of life.
Don't forget to play!