In 1998, it was May 6th through 9th, so I don't think we can really assume mid-May. Also, in 2005, it was Thursday through Sunday, while previous recent Massey stands were Wed (or Thurs) through Saturday.
If I were you, I'd keep flexible for the whole month of May. Saturday the 3rd is already booked (Ron Sexsmith), so that's the only week that it can't be.
I'll post the dates at my web site
gordonlightfoot.com as soon as I hear about them, and I will also send out a mailing, so sign up for my mailing list if you want notification. And I'm sure it will be posted here by me, Char, or anyone else who gets the word early.
Stay tuned!