I'm only on my fifth week so I don't know a lot. I'd personally have minimum 5 syns each day anyway then have a flexisyn day. I just think if you really limit yourself you are more likely to go overboard. Also you will be using your bday food as a reward which I personally feel is the wrong attitude for me as I got to my weight rewarding myself with food.
My baby's 1st bday was on my second week and we went out for a meal, I ate what I wanted without sw crossing my mind and I had a bigger loss than my first week. Looking back I still made healthier choices than normal ensuring I had plenty of veg and had a smaller wedge of cake instead. I hope whatever you go for works well for you and try not to worry too much about it on your special day xx