The first two weeks of May are the ideal time to seed canola, based on long-term yield results from the Prairies. This has been shown in Canola Council of Canada research as well as crop insurance results. (See links below.)
Yes, there is a frost risk, but this risk is considerably lower now than in late April. Links to frost risk maps.
On frost: Frost prior to emergence is not necessarily a concern. Instead, note the weather outlook and frost risk for a week from the seeding date. That is most likely when crop emergence will begin.
Alberta data showing effect of seeding date on canola yields
Manitoba crop insurance data comparing yield by seeding date
Canola Encyclopedia chapter on “Time of seeding”
CCC factsheet: Seeding canola the ideal timing