Generally when the Canadian govt creates a new law, and places a date on that law, the change would become effective as per date, which in this case is Dec. 1st 2018. That has been my experience when working with some of these governing bodies, such as the CFIA.
To add to that, from the link on the new regulations that I previously posted:
Timelines for the changes
Drugs listed in the
prescription status: affected drugs and dosage forms will continue to be available without a prescription until December 1, 2018.
After December 1, 2018, these drugs will only be sold if there is a prescription from a veterinarian.