Honestly? I wanted to pick two points in the year that were about six months apart and easy to find on a calendar. I could just as easily go with the Equinoxes or opposing sets of cross-quarter days; solstices were just the first that came to mind.

...Come to think of it, the winter solstice would probably not be a great time of year for a mating run. Maybe equinoxes or the Nov/May cross quarter days, then.