I'll add a few recollections to Damian's post.

Mike Stevens/Stephens Club 800 began playing "rock" music in the spring of 1956 following on the heels of Elvis' national  TV debut in Jan 1956. Apparently Club 800 existed from the late 1940s  targeting the after school crowd with pre-rock music they thought young people favored. Club 800 featured in studio reps from various Montreal high schools. The out of studio weekend show/dance I remember most was at the  Pointe St Charles Boys and Girls club. This was a big deal - Mike frequently brought in NYC R&B acts who were playing local clubs eg The Esquire Show Bar. My own high school band, The Bristols, performed on the same stage as one of these "name" acts. That was the pinnacle of our career. 

Around the same time as Club 800,  CKVL's Palmares American played rock charts starting at 6:30PM to 7:30PM or 8PM each weekday. Hosted by Leon Lachance.

These two shows were pretty much all us young folks had on radio until CKGM went full time Rock in 1959. I think CKGM still  included adult pop along with the young peoples' rock music.

One other short lived broadcast occurred before 1959. A small station in Cowansvile played early evening rock in 1957 and 1958. Reception was tricky in Montreal. I remember that  station  for debuting  two songs in particular - Raunchy by Bill Justis and Rumble by Link Wray.

Montrealers  with antennas were able to watch American Bandstand from Poland Spring Maine starting (probably) in 1957.

I don't think the Plattsburg and Burlington radio stations offered Rock that early. They may have started in 1959 but by them we had CKGM.