When was your birthday?
It is correct say it like that when you're asking when the birthday was yesterday?

Question

When was your birthday? 
It is correct say it like that when you're asking when the birthday was yesterday?

Answer

Yes, that is the correct way to ask the question.

Answer

Yes, you can  also say: My birthday was yesterday or Was it his birthday yesterday? or Yesterday it was my/his/her birthday.

Answer

Yes. You can say, "When was your birthday?"   You could also ask, "Was your birthday yesterday?"

Answer

Really, my birthday was yesterday "12th september"