I know I've mentioned this in previous posts, but I am definitely a big supporter of the CHEAT DAY! Taking a cheat day, or even just a cheat meal, once a week is a way to keep your body guessing and give you a break from your (hopefully) normally regimented diet. Since your body can adjust to pretty much anything, the cheat day allows you to surprise your body and change things up a bit. It's also a nice chance for you to enjoy some of the foods you wouldn't normally eat as part of your weight loss plan. I'm not saying healthy foods can't be delicious, but it's hard to get excited about veggies and lean proteins all day, every day, for weeks on end. Since a cheat day lets you indulge in some of your less healthy preferred eats, it keeps things interesting for you and makes you more likely to stick with your long term plan.
It's important to remember that you can't use the cheat day as an excuse to eat whatever you want, whenever you want. For example, a friend invites you out to dinner and you decide, "Sure, I'll spring for the burger with everything on top and the chili cheese fries, this can be my cheat meal!" Then 2 days later someone brings cupcakes in to work and all of the sudden, you're having another cheat meal, and another, and another. That kind of thinking can snowball and quickly ruin all your hard work. You need to plan this ahead of time. Pick one day a week when you know you're going to be a little more lax with your food choices, and keep it the same day every week. For me, my cheat day is always on the weekend, since it's easier for me to keep a regimented meal plan during the week on a full time work schedule. Also, you're more likely to be going out with friends to dinner and drinks etc. on the weekends, which is when you want to loosen up and be more lax about your choices.
Cheat days, while enjoyable, can also be very dangerous for people who don't have the right amount of self-control. If you're the kind of person with the "once you pop, you just can't stop" mentality, then taking a cheat day might not be the best idea for you. For some people, once they've cheated it's hard to then go back to their healthy meal plan and they just give it up completely. If a weekly cheat day is too frequent for you, it might be better to time your cheat day with milestones, like for every x amount of pounds you lose, or once you're able to run x miles. That way your cheat meal isn't quite as frequent, but you still have something to look forward to along the way.
Also really important: even on your cheat day, always remember that ultimately, you have set weight loss goals for yourself and you can ruin them by making poor food choices. The cheat day should not be a free pass to go crazy and eat things that will completely undo all your fabulous progress. Keep your portion sizes in check, and try to still include fruits and veggies along with the more unhealthy things you might be enjoying that day.
Most importantly, do what works best for you. What works for some does not work for everyone. Know your limits and stay in control of your food, your choices, and your goals. Enjoy!