Celebrate whenever you want.  The Queen does it.

A person can celebrate their birthday whenever they wish or not celebrate their birthday at all. It has no impact on their real birthday one way or the other.

No, you cannot change your date of birth.  How many errors to you think there are on birth certificates?  I really don't care when you celebrate your birthday.  You've posted this several times.  What EXACTLY is confusing to you?  I already gave you permission to celebrate on another day.  What more can I do?

There are no birthday police.    You can celebrate your birthday every day of the year if you want to.

Correct that you cannot legally change your birth date.

I actually have a friend who was born in a Muslim area of Africa and since they don't celebrate birthdays, she had no clue what hers was.  The USA government gave her a birthday (January 1). 

Sure it is .  Why would anyone mind ?

You "celebrate" your birthday any time you want.