Autumn 2018 begins with the autumnal equinox.. ... in Northern Hemisphere will begin on this Saturday, September 22... Autumn 2018 ends on Friday, December 21
More specifically, the start of each season is marked by either a solstice (for winter and summer) or an equinox (for spring and autumn).
Winter 2018 Begins on Friday, December 21 and ends on Wednesday, March 20, 2019