It's usually the second weekend in May, and details are posted here:
http://www.thundersprint.com/
Weatherwise this year was a bit of a disaster after noon, but it's a fun day none the less. Fascinating to see Jim Redman mingling with the crowds - 90 per cent of whom oblivious to his presence, and 10 per cent just gobsmacked that one of the gods had appeared on earth!
William
EDIT: Whoops! Didn't spot that the first poster had already given a pointer to the website. I must engage brain before replying next time!