How do you celebrate your birthday whilst dieting?

Here's what I would do: Fill yourself with healthy foods as much as possible. Eat a healthy breakfast, even if in bed. Add some flowers to the table or tray. Treat yourself like a queen who loves healthy foods. Skip the bacon!

Lunch doesn't have to be sugar loaded. Eat a healthy meal and have some fruit for dessert.
Eat cake mid afternoon.

Dinner? Whatever the heck you want since it's YOUR DAY.

Just know that whatever you do, you are going to counter it with added commitment the next day. A bit more exercise perhaps.
Don't starve yourself so that you can binge later -- it never works.