RE: Eat too much one day, eat less the next?
Excess calories over your planned ammount will (if dieting) probably top up glycogen stores and the body will use these for fuel as preference if in high quantitys.
When to you less the next day, the body will utilise this glycogen that was stored.
If bulking, glycogen stores may already be full and fat storage would occur. The next day if eating over excess again, more fat storage occurs.
Reducing calories would promote glycogen use and perhaps some fat burning i suppose.
As said, if you eat to much then ah well - Get back to the plan and ignore it.