Yesterday, June 21, the planet Earth —like a spaceship as it's usually called because it travels across the space— was touching one of the four singular points in its elliptical trajectory around the Sun. Some places experience the longest day on it, it's called the summer solstice. There are two, in winter and another, then, in summer. They are reversed one from another.
The orbital path from each limit —the most distant points in the ellipse— also marks two middle points. These are the equinoxes. The prefix equi- comes from Latin "aequus" that means equal. Well, it's incredible that in this day, you could measure the same quantity of luminosity and darkness. This also happens also in the Ecuador, every day of the year.
As it be, the four mathematical reasons to mark the beginning of each season are correct because the orbital ellipse has two limits and two equidistant points to them. That's all the info that you would need to understand why the seasons start on a specific day!