It's like saying, on the date I was born rather than in the date I was born. :3
I'm getting a pony on my birthday vs. I'm getting a pony in my birthday.
- I'm turning sixteen on my birthday.
- I really want to go out of town on my birthday.
You can typically just tell from how it sounds. :)
On my birthday I'm going to the club.
In my birthday party we had cake.