My husband was "banned for life" for entry into Canada for having marijuana on him back in the 90's. We had completely forgotten about this when we traveled to Toronto some years back, but were reminded at the border where he had to sign some paper that indicated he could be jailed if he returned to Canada. We have a family wedding in the near future that will take place in Canada. Now that we have legalized Marijuana in Michigan as well as Ontario, how can we get the ban lifted?
Get an immigration attorney, a competent one. This is not the kind of question for which you solicit answers from random guys on the internet. It is not a trivial matter.
Typically the fact that the law has changed does not mean immigration violations of that law in the past are forgiven because the issue is it implies you do not have a problem breaking the law, so you’re viewed as more inclined to break other laws.
You might want to start by reading about Individual Rehabilitation (permanent) and Temporary Resident Permits, which can be used to enter Canada after a conviction.
If the crime committed would have a maximum penalty over 10 years in Canada, you'll probably have to go the route of Temporary Resident Permit.