This year summer started May 13: that's the first day we ate food from the garden.
When did spring start? Back in March, when I started planting my garden beds.
When will fall start? I'm not sure. Perhaps when the last of the blackberries are gone from the vine.
Winter? I know it's winter when I don't even try to go outside after work.
(On a related note, despite putting up quarts and quarts of blackberry jam, we are going to run out before Blackberry Campaign 2008 commences: we opened our last jar yesterday. After that, it's two jars of cranberry sauce, then pfft.)