When can we plant our Heirlooms?

Last Frost
  Around here, May 24th weekend is usually considered to be the day we plant annuals.  We do this because statistically the threat of frost is lower.  Below is a frost map from the Ministry of Agriculture roughly corresponding to Ontario's climate zones.

Zone	Frost-Free Period  (Average in Days)	Average Date of  Last Spring Frost	Average Date of  First Fall Frost
A	170-190	April 25	October 20
B	160-170	April 30	October 13
C	150-170	May 3	October 8
D	130-165	May 11	October 1
E	125-145	May 17	September 26
F	115-125	May 24	September 22
G	100-115	May 27	September 17
H	100-110	June 3	September 16
I	90-100	June 7	September 9

Now these are just averages.  The department of Agriculture goes onto explain that there will be 50% chance of frost on the dates stated in the above table.  (1 in 2 years there will be frost on that date)  It then decreases to a 25% chance 7 days after the list date, dropping again to a 10% chance, 14 days from the date. (full article) So, can we plant our tomatoes on the May 24 weekend... yes, statistically we should be ok.  We do need to be carful to watch for a chance of frost for the following 2 weeks after May 24th.  I do seem to remember a late frost a few years back on June 4th.
The work around frost:  For a small amount of money you can purchase a frost protector for your heirloom.  The basic concept is once filled with water, your protector will act as a greenhouse keeping the plant and soil warm and sheltered from the elements.  These protectors are said to extend the growing season of your annuals by up to 8 weeks.
Last frost is not the only concern when planting your heirloom.