Please how can i recover my date of birth?

Use the contact support link at the top of the page, select your iPhone, select Apple ID and iCloud, scroll down and select forgotten Apple ID and security Questions, then select the option to reset your security questions. Follow the instructions to change the security questions.

What happens when you try to enter your birthday?

Recover it from where and for what? How did you forget your birthday?

There is an option too if you forgot your passcode. Your passcode should be the same one you used to get on this forum.