Last July I asked my dad why people say "Happy July 4th" instead of "Happy Independence Day"? Dad said every day in America is Independence Day, because we integrate independence into our identity so deeply that we can't imagine acting specifically independently on a particular day.
I think people say July 4th because "Independence Day" has two more syllables.
Okay, I'm done thinking. Off to work out and buy groceries and eat food off my friend's grill. Woot!