Joanne, this is a perennially-interesting subject for me. As a complete newbie to New England gardening five years ago (moving from Monterey on the central coast California where it MIGHT freeze once every 4-5 years!) I have scouted freeze-data lists ever since I moved here, trying to get some hard-and-fast rules that I could understand. Plus, it seems to vary on Cape Cod from town to town! The website below gives a lot of information for Massachusetts ("Hyannis" is our Cape Cod reference-point) but I still find the probability rankings confusing. Here's what I'm finally come up with in our 7a zone, after five years of asking local gardeners and observing my own plantings:
The average last date of possible frost is May 25. You can start hardening-off seedlings in a cold-frame or with other protection but things don't really start to grow in the ground until the first week of June.
We have a long and relatively warm Fall, but you will have a frosty night for sure by the end of October.
Condensed version: just remember "Memorial Day" and "Halloween."
Edited to add that that makes me about 17 weeks before last frost date as well!
http://cdo.ncdc.noaa.gov/clima...
