We’ve been collecting temperature data from our weather station for some years now.
We’ve analyzed this to determine the number of days our growing season is.
Some crops can survive -3 deg C “heavy frost” nights while most can survive -1 deg C “light frost” nights.

Actual Growing days
Year #days between light frost #days between heavy frost
2003 172 Apr30-Oct23 194 Apr24-Nov08
2004 156 May08-Oct11 201 Apr16-Nov03
2005 156 May17-Oct20 215 Mar13-Oct21
2006 no data
2007 147 May18-Oct12 153 May13-Oct13
2008 154 May05-Oct06 160 Apr30-Oct07
2009 129 May13-Sep19 179 Apr17-Oct12
2010 119 Jun13-Oct10 142 Jun11-Nov01
2011 167 Apr22-Oct06 167 Apr22-Oct06
To date we had -3 “heavy frost” last night, April 19th, 2012.

A note that we are far from Lake Ontario, which is a huge moderating influence on temperature in the spring and fall. Hence our growing season is much shorter than say, Kingston Ontario. Typically in the summer, days are +5deg C and nights are -5deg C over those in Kingston.
This in part comes from very thin soil, a lot of exposed limestone, and the Canadian Shield just a short distance north of us.