Being a Torontonian, I'm a regular visitor to the St. Lawrence Market. Visitors should be aware that there are technically two markets: the South Market and the North Market.
The South Market is the original market, open most days (if I remember correctly, Sunday and Monday are closed days). Great place to shop for food and in the lower level, more food and trinkets.
The North Market is the newer market, and is more of a multi-purpose facility.
Saturday is a "food market" day - farmers and other merchants offering the best produce, meats, cheeses, etc. from Ontario. Just get there early...by about 9 am the place is an absolute zoo!
Sunday is a "flea market" day - lots of nick-knacks, antiques and memorabilia on sale. Well worth a visit as well, if just to people watch.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC