I'll be in Toronto for Victoria Day. Will most places be closed for the holiday as they are in the US for, say, Memorial or Labor Days?

If by "most places" you mean stores, yes, they likely will be closed. Many restaurants will be open, though.

Here in Calgary, a public holiday like Victoria Day or Labour Day is like a Sunday. Banks are closed. Restaurants and tourist attractions are open. Public transportation operates on a Sunday schedule, which represents a reduction of service compared with a week day. Stores in small plazas are closed. Stores in large malls follow a Sunday schedule, which in many cases means being open from noon to 5 pm.

I suspect, but don't know for sure, that this pattern is common across Canada.

No Judy, the pattern is NOT common across Canada. In Ontario, the only shopping open, beyond the usual convenience stores and pharmacies, on statutory holidays are those located in areas of touristic interest. In Ottawa, that means Rideau Centre/By Ward market. What it might mean in T.O.I have no idea.

Thanks for the clarification, Ron.