It celebrates the creation of the Dominion of Canada through the British North America Act 1867, which came into effect on July 1, 1867, uniting three British territories — the Province of Canada (southern Ontario and southern Quebec), Nova Scotia, and New Brunswick — into a federation.
The holiday itself was formally established in 1879 and was originally called Dominion Day, making reference to the Canadian-originated term 'dominion' to describe the political union, at a time when the Fathers of Confederation were hesitant to use a name such as the Kingdom of Canada. The name was changed to Canada Day on October 27, 1982.
I didn't know about this holiday until today. And since I'm going to Toronto in a few weeks, I guess July is going to be Canada month for me. Maybe I'll put that Canadian sticker on my car, to show solidarity with our great neighbor to the north.