I wondered about this too. Last summer when the Magic was in the Med. I tried to ask about fireworks for the 4th. I got the impression that the fireworks were Pirate night regardless of when the 4th of July fell. For the June 29 cruise, I believe that would be July 5th (for Pirates night). And in watching all different Med cruise meet threads I have come to appreciate how diverse the demographics are of guests from various countries, so maybe it's easier for them all around to stick to the schedule.
I learned something new here, I don't remember knowing that Canada day is July 1st. I know that your Thanksgiving Day is in October because it is near the day we celebrate Columbus Day in the U.S.