Alcohol

have fun at your festival!
Make sure you drink plenty of water, ideally inbetween each alcoholic drink to stay hydrated. you are going to be bouncing around and moving lots, so i would will burn more than average calories, but try not to go overboard. you will save calories (and cash!) by limiting the drinks you buy.
Before you go, look up how much your favorite drinks weight up in terms of calories. that might help you make healthier decisions. also, bear in mind that increase alcohol may decrease your willpower for resisting the food outlets there will be, so avoid them (the smell of doughnuts has been my undoing before!). Stock up on food bars like fruit and nut snack bars, or cereal bars to take with you, they are much easier to track for calories (keep the empty packets in a pocket and tally them up when you get a chance) and though they are not the most healthy, they will be better than a takeaway meal loaded with fat and sugar and salt! Also much cheaper!
Most import though, have fun. A festival is a rare event, and you should enjoy it. Try not to stress too much about staying on track, and as long as you make an effort towards not drinking too much or eating too much junk food i do not think you'll put on 5 kg over one weekend. The scales might show an increase in your weight, but you've been losing the weight so far, and you will be able to lose any excess again after the festival.

Unless you have high, uncontrolled triglycerides, just HAVE FUN! And drink extra for me!