Enjoy that cake!
Planned and deliberate weight loss takes time. How much weight you want to lose will depend on just how much time it will take.
One of the problems is, just when to start that weight loss plan. Next week might be your birthday, it might be Christmas in a few weeks time, there might be a string of weddings and family parties, where sticking to that diet will be difficult.
This is one reason why people keep putting their diet off. There is just never a good time to start.
Well, you will need to change your thinking a bit. If it is your birthday, enjoy that cake. It doesn't mean that you need to starve yourself the day before to compensate, that would be both daft and also counterproductive.
The reality is you need to build into your diet plan the days that you won't won't be sticking to the weight. A decent diet doesn't just account for losing the weight, but also for the days that some weight is put back on. There will be those days, and you need to account for them.
So, if it is your birthday, enjoy your birthday and eat that cake. And please, don't make endless references to how you are on a diet, you know, the things that people say, "Well, there goes the diet", "I'm supposed to be dieting" and so on. Not only is it really rather boring for everyone to listen to but it doesn't help anything. Believe me, if you are that person who says these things, you will be saying this a lot, and everyone else is just too polite to tell you to stop it! So I am telling you. Please stop it.
Your weight loss plan should allow for one day a week where you can eat whatever you like. There are physiological reasons why you should have a day off a week from your diet, and it does wonders for your psychology too. It is OK to indulge, just not every day. Plan for this.