The Earth takes about a year to revolve around the sun. Within that year the four seasons arise: summer, autumn, winter and spring. According to the climatic classification, autumn has already started on September 1 and the season lasts until November 30. For the meteorologists, autumn starts on September 1.
Around September 23, the astronomical autumn begins. Fall officially starts on September 21. Around September 23, the sun is exactly above the equator, so that day and night have the same length everywhere on earth. Therefore, autumn usually begins on September 22 or 23. Officially, the fall runs from September 21 to December 20. (source KNMI)