I was just wondering, was it randomly chosen or...
sean' and yoko' have it right, but the dates of the solstices were historically considered the middle of summer/winter instead of the beginning (as they are listed on modern calendars). This can be demonstrated by the English names of the holidays celebrated on those dates - Midsummer and Midwinter. Not a clue why modern calendars have it different. The old way makes much more sense.
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Originally? AFAIK by the longest and shortest day, ie, the most and least amount of time the sun stays in the sky, probably collated over generations.
The seasons are based on the soltices, which are the days when the sun shines the longest and shortest. The dates are very exact, and several ancient structures such as stonehenge, egyptian pyramids, and native american landmarks are believed to have their design based around these dates.