On the BFL program, Sunday is supposed to be your free day. In the new journal by Bill Phillips, there isn't a seventh day because you always do your 7th, 14th, etc. day as your free day.
In real life, free days are very useful for if your company dinner at the most expensive and high-fat restaurant in town is being held on a Wednesday. Or your best friend flies in for one day and you want to take her to all the hot restaurants in town.
We schedule ours for Saturday usually. Even though I teach Sat mornings, I'm up early enough I can go get breakfast at McD's or Burger King. Since I love eating breakfast out, but that's not a common practice, it makes it two treats in one. We originally started with Saturdays, before my class this semester, because if our free day was Sunday, we couldn't eat anything for two to three hours while we were at church. My husband said he wanted to be able to pig out all day long.
The weekend before Christmas we are switching to Sunday, though, because my family is having their Christmas dinner together that day and I want all the Mexican steak I can eat. (We only have turkey if a bunch of the in-laws are coming.)
The idea behind the free day is that if you know you can eat anything you want in just a few days, it will be much easier to eat according to the plan. And while some people worry that they will eat as many calories as they've lost, that's generally hard to do. Plus, after about the second week, we found we didn't eat nearly as much junk on our free day because we felt sick whenever we ate high fat, low protein food. So the first week or two I think I had like 8.000 calories, which is enough to add pounds, and 150 grams of fat. But after that, my calorie count never got over 2.500.