I live on the west end, in south Etobicoke, so places like X-Planet Games (Burnhamthorpe and Tomken, Mississauga) and Heavy Support Games (Islington and Queensway, Toronto) are my "ideal" locations, but that's just because I live in between them. 401 I go to sometimes to pick things up, so I have no problem going there. It's an easy drive/subway trip for me to get to.
I don't really have a "best day", but I can make anything work with the right kind of notice. The real issue with organizing tournaments, in my opinion, is coming up with some kind of prize support to get people interested. Otherwise, it's just a glorified game night. Not that I'm opposed to that, but winning something is always nice.
If you're planning on organizing it, talk to the store and see if they can offer some kind of store credit as a prize. Charge everyone a small fee to enter, and put that towards gift cards, and split the pot between the top 2-4 players.
By example, everyone seems to have responded really well to the Kessel Run tournaments for X-Wing. Not only are free ships a neat prize, but getting them well ahead of the proper retail release is even more attractive; while scoring a TIE Interceptor when you already have 4 isn't a bad thing, it's not as awesome as getting one when you couldn't even get any at all.
For Star Wars, giving out Force Packs (unless FFG provided an early release like the Kessel Run) would be a weak idea, because anyone playing in the tournament would more than likely have one already, for the purposes of making their deck, or at least having it in their collection. Also, because of the card distribution model, for a lot of people, a single Force Pack would be all they need.
You could also opt for straight cash, but I think the store would be more likely to co-operate if you used the admission fees to ultimately convert the money into store credit, which is cash in their pocket.
In any case, count me in for whatever you're planning.