Q: because if we do that then why don't we call christmas the 25th of december?
A: It's funny how things evolve. When you say December 25th or February 14th or even March 17th, people know what holiday you are talking about. 4th of July as a phrase comes from the song Yankee Doodle Dandy - "a real live nephew of my Uncle Sam, born on the 4th of July!" so maybe that's how it got into circulation.