You could do that, but would it not leave you feeling more deprived throughout the week and therefore more likely to binge and eat over-excessively and mindlessly? Surely this is not healthy.
This type of occasion is what exactly what flexi-syns are designed for. If you have a look through your pack or do a quick search on here you'll find lots of threads explaining exactly how flexi synning works. It might not garauntee a loss, but it would limit the damage and ensure that you remain in control.