I wouldn't believe anybody who says that they don't have those days. Rather than beating yourself up, think about it this way - it is normal to indulge on occasion, and didn't you get the band to be normal?
Maybe it would help to think this way - eat really well 80 or 90% of the time, so that you can let loose every once in a while.
Embrace what happened and let it go. One of the worst things you can do to yourself is to get in that cyle of negative self talk which eventually can lead to "oh the heck with it all, I blew it so I might as well eat anything and everything I can find" (or is it just me that can get into that kind of thinking if I let myself?)