I think the original Square Foot Gardening book by Mel Bartholomew has a lot of charts in it about when to start vegetables. Although aesthetically I am not a fan, he's worked out a lot of information. You might find it in your library. If not, you might find some other source by Rodale or Burpee etc. He has a new one coming out but I don't know what's in it obviously.

Here is a link that might be useful: Square Foot Gardening

Andy, do a Google search for "Frost zone climate maps" and you will receive an assortment of sights to choose from. I'm a little west of New Brunswick, NJ and our last average date for frost is Apr. 30th. First average date for frost is Oct. 20th. May 1st is fine for planting corn, beans, cukes, squash etc. by seed. Toms and pepps transplant on May 15th., two weeks earlier if using protection (cold caps, waterwalls etc.)....JJ61

I'm in central Somerset County Z6. Last year I started my pepper seed Feb 20 and my tomatoes scattered dates between Mar 4 and Mar 13. All got planted out around May 15. My Carrots and lettuce were direct seeded April 9, but several people I know start earlier than that for lettuce.

Visit the Master Gardenrs of Mercer County web site and you will find a month by month calendar telling you what to do that month in terms of gardening.

http://www.mgofmc.org/monthlytips.html

Here is a link that might be useful: Master Garderns of Mercer County monthly tips

You've probably already figured this one out, but for anyone googling in the future (like I did!) here's the diagram for Zone 6 from Veggieharvest.com

Here is a link that might be useful: Zone 6 veggie planting calendar