Traditionally, most people think of the seasons as Autumn is September, October & November, etc.. however, if we want to get really technical the seasons change because of the sun..
Why do we get seasons?
Planet earth is on a tilt. Therefore different regions are pointed closer to the sun that others. It takes 365 days to orbit the sun.
In June, the north pole is pointed more towards the sun, therefore June, July & August are warm in the northern Hemisphere.
The tilt does not change at all, merely the location of the hemispheres.
When are the seasons?
Spring: Spring Equinox till Summer solstice
Summer: Summer Solstice till Autumn Equinox
Autumn: Autumn Equinox till Winter Solstice
Winter: Winter Solstice till Spring Equinox.
Spring Equinox: March 20-22
Summer: June 20-22
Autumn: September 20-22
Winter: December 20-22
The sun shines directly onto the equator, and the night and day have equal length. There are two every year, Spring and Autumn.
The sun shines the furthermost away from the equator. The Summer Solstice is the longest day of the year and the Winter solstice is the shortest.
I personally, abide by the seasonal calendar. The weather does not simply change overnight, but it gives a good indication of whats going on with mother earth., and our universe.