I'd be just as miffed and hacked off as you, have your day off as planned and enjoy it. Those scales play nasty tricks, and you know it's not fat gain but we focus so much on a machine it's hard not to be annoyed. Don't cry over a false gain, that's seriously when I knew I had to give up the scales as how could a false number make me soo depressed sooo early on in the day?
The scales can't tell you how much is fat, how much is poo, how much is water, how much is muscle, how much is the weight of the food still undigested in your tum, it only tells us an overall guide at that time, on that day. A pee, it could be gone. 1ltre of water weighs 2.2lbs.... If you were to weigh after drinking 2litres you'd be 4.5lbs up but no fatter?
Sorry your feeling so down, big hugs to you xxx