Neither is correct. We say: On the eve of his birthday.....

My daddy spent most of his school years having idiots answer his questions of "why", with, "Just because". Miowww I'll get my daddy to message you and answer your questions. If he wasn't banned everyone else could read them too, go italki!!! Keep the idiot and ban the helper! Cheeee singggg ahhhhhhhhhhhh Miow *fart*

You can say "on the eve of my birthday, or on the eve of eve of any other day " ( like Independence day, farther's day.. etc ), and it would be right. It's also right to say "on the eve of Christmas, or on the eve of new year". "Christmas Eve " and "New year Eve" are the events celebrated in the evenings, and have come to be known as such.

We don't use Birthday eve :) The only two words in English (I think) that are spoken this way are : Christmas Eve New Year's Eve There is no reason to it. But in a language there are certain things that are spoken and there are certain things that just aren't :) If I were the creator of the language, I would have made it easier. Unfortunately, I have no power over English ^_^