If you consider to book your ticket based on this KLM promotional sale please look for travel dates till end of June 2015. (based on date of your outbound flight to North America). We recommend to use service of Skyscanner to quickly find out which airlines are available from your hometown. Please refer to below booking samples if interested in this deal.
Booking samples - look for return flights from Antwerp (ZWE) or Brussels (ZYR)
Belgium - Montreal from €346
Belgium - Montreal from €334
Belgium - Montreal from £249
Travel date samples - The easiest way is to search directly via KLM web site. They offer +- 3 days view so you can easily select travel dates that suit you the most.
Booking sample of cheap Return flights from Belgium to Canada (Montreal) for €334 here. In this case you would be departing from Antwerp to Montreal on 24th of June. You would be then returning back from North America to Europe on 13th of July 2015.