By Mid August I start noticing that overnight lows that once stayed consistently above 60F are now mid 50s and the far northern parts of Canada/Mountains start getting the first sub 40F. I cheer/day dream about the upcoming fall in mid august when the averages quickly start diving.
Long story short Winter-November 15th to February 15th in my mind, Spring February 15th-May 15th, Summer May 15th-August 15th and Fall August 15th-November 15th
Its a weird way to look at it but at the start of each of those dates, even though firmly entrenched in their season the first signs of the upcoming one begin appearing.