First of all, let's start with the idea that if we are going to have four seasons that they should be split equally with each one containing roughly 90 days.

I think that one of the best arguments that could be made for when summer should start and end is that premise that summer should be centered on the 90 days when the sun is "highest in the sky" and when you can most easily get suntanned (or burnt). That means that summer should start about 45 days before the summer solstice which occurs generally around June 21st or 22nd each year and end approximately 45 days after that date.

If you do that then you come up with a summer season that lasts from about May 7th until about August 6th because those are the 90 days during which you get the most intense sun here in the Northern Hemisphere.

Tomorrow we look at temperatures.