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1.
The yearly celebration of a person's first time trying or enjoying cannabis in some form. Sometimes celebrated by sharing one's cannabis or paraphernalia with members of their smoking community (reddit.com/trees, for example). Can also be celebrated with cannabis-infused space cake with candles.

Also, just another reason to get together with your stoner friends and get baked. Not to be confused with 4/20, the annual holiday that celebrates pot culture in general.
Miles invites his stoner circle over to his house every year at 4:20 on July 5th to celebrate his canniversary with them. They usually bring him gifts of rolling papers, a dub sack, and next year, they're going to Amsterdam for his canniversary!
by Milesabove July 05, 2011
 
2.
A year from the date on which you were fired from a job.
"Next week is my canniversary from Enron!"
by seoladair February 05, 2009