Opening of season mid November - early December: Not Recommended
Closing of season mid April - mid May: Recommended (But need to consider which mountain
to ski)
Most ski resorts in the Canadian Rockies open mid November and close mid to late May, with generally the best conditions from mid Feb to mid April. Now, don't think that because the resorts close in May that it's because of lack of snow. No: it's due to a lack of skiers and riders from the local area or they may be in a National Park and the Bears are waking up, and skiers and bears don't mix. The snow is still great!
Some resorts, particularly around the Banff area boast of their best days being in April, with longer sunnier blue sky days, and a great snow base. They often close with a 3-4 meter base!