A meteorology definition of when spring becomes summer is when the daily mean temperature over a week reaches 10 degrees Celsius.
Here in the Netherlands the summer starts on the summer solstice, around the 22nd of june.
That's the astronomical summer, and the most used term of 'summer' when speaking of dates.
But I think the whole idea with saying 'late spring' is to not make any commitments to dates. =)
Here March 1st is the start of autumn and June 1st is the start of winter. But don't 2K (and American in general) go with the solstice? I seem to recall that from last time around.