OK tnx a millionAccording to a Google search I've done it appears to be a day when you temporarily abandon the diet that you're on, and eat what you want for the day.
Judging by the expression on the guy's face in the Instagram photo, I'd assume that fits the context well enough!
Tnx a lotFrom Urban dictionary Urban Dictionary: Cheatday: cheat day
"A day on which you are allowed to break your diet, as complete abstinence is impossible for most people. It also provides a reward for the sticking to your nutrition plan over the rest of the week. The "cheat" can be small (e.g. a chocolate muffin with lunch), large (e.g. a fast food meal), or systemic (e.g. Full Fat Friday).
Carol: Oh my god, Jenny! I can't believe you're eating cheese!
Jenny: It's ok, Carol. Today is my cheat day. "