Jumping the gun
Well, in my haste to get the season started, I planted lots of tomatoes and various squash seeds way back in late February. With the result that they were ready to be planted out in April/early May, which was really a bit too early for them. We had a spectacular April in terms of heat and sunshine, but when they got planted out (and the tomatoes were quite leggy at that stage) the combination of winds, and temperatures below optimal have resulted in burned leaves, and plants that didn't really thrive. I'm now working on my second batch of seedlings. Hopefully they will fare better.
Note for next year - if I'm driven to plant stuff early, go for lettuce, onions, broccoli, cabbage, beets, mizuna, peas, carrots, celeriac, thyme, parsley.
Wait till March for planting the soft mediterranean plants like tomatoes and squash.