Well. It originates from the seigneurial period, when seigneurs (lords) had the power to evict farmers whenever they wanted. To combat eviction that would leave families destitute and in teh snow, the lords were banned from evicting families before the snow had melted. This was written into law, and the law evolved to cover rental leases, with a lease term for yearly lease starting on May 1st.
Later on, because this meant all the kids in the province moving in the middle of school year, we abolished the mandatory term for yearly leases, but also lengthened all lease for that year two months to put the end term out of school year territory.
The reason people have kept with the old lease is just plain simplicity. Tennants and landlord who had yearly lease going that ended on July 1st renewed them for another years. Because so many leases were ending then, this is also the time of the year when both offer and demand reached peaks, so lots of new leases were signed to start on July 1st because, that was when people (whose old lease had ended on July 1st) needed new ones.
And while there is an increasing number of people with yearly leases that begin or end on another date, the cycle has gone on and on - July 1st is when both offer (due to people leaving their old places at the end of their lease) and demand (due to people having left their old places needing new ones) are at their highest, so naturally the time of the year when most leases tend to be signed.
All joking about moving day aside, though, to me, I've never done moving day on July 1st, and I've celebrated Canada day a few times, mostly while in Ottawa (which I'm not this year). But mostly, for me, July 1st is the day before my birthday
