The long weekend in May typically signals the start of summer, but Enviroment Canada says "not so fast" for many parts of the region.
Temperatures are expected to go below zero overnight.
This is a warning that at or near zero ground temperatures will cause frost in parts of these regions.
Both air and ground temperatures are expected to go below zero tonight. Frost is expected to be widespread.
Link: http://weather.gc.ca/warnings/report_e.html?nb7
Correction: it's not called Environment Canada any more. related story: http://elizabethmaymp.ca/news/publications/press-releases/2013/04/17/environment-canada-disappears-from-government-of-canada-site/
3 court briefs from August 22 and 23