Why? Because I say so and I really like autumn. That's why.
There are those who say that solstices and equinoxes mark the beginning of each season. In which case we're still three weeks away from autumn. Others say that solstices and equinoxes mark the midpoint of each season.
Nonsense, I say! Solstices and equinoxes have to do with the alignment of Earth's tilt to the Sun. Seasons have to do with the weather and the temperature. The two are related, but not the same.
If you say solstices mark the beginning of seasons, then a week before Christmas isn't winter yet, which seems kind of ridiculous. If you say equinoxes mark the midpoint, then Valentine's Day is already spring, which is even more ridiculous.
And more importantly, there is no first day of any season. As my brother and I were discussing on Facebook, everything is continuous, and seasons are a prime example of that. It's not like there is one single day where the temperature drops from 30°C to 20°C and all the trees change color. It's a gradual transition, like red turning to orange in a rainbow. Any line of demarcation is going to have to be arbitrary.
And since it has to be arbitrary, it might as well line up with another, well established arbitrary date-point. In this case, the first of the month.
And so, I decree: The first day of autumn is September 1st. By extension, the first day of winter is December 1st, the first day of spring is March 1st and the first day of summer is June 1st. And it lines up much better with the weather that way.