In meteorological terms, 1 September is the first day of Autumn.
Meteorologists define Autumn as the months of September, October and November. But, in astronomical terms, Autumn starts at the Autumnal Equinox, when the Sun crosses the equator going south; this year it will occur at 0254 UTC (~GMT) on 23 September.
The Sun will reach its furthest point south of 23.4ºS at the the Winter Solstice - 2223 UTC on 21 December; the beginning of astronomical Winter.
Because the Earth takes ~365.25 days to circle the Sun, the equinoxes and solstices occur about 6 hrs later each year and, to keep dates consistent year-on-year, a leap year of 366 days is introduced every 4th year; the next is in 2020.
Since 2002, UK has aligned itself with Europe by adopting British Summer Time (BST) between the last Sundays in March and October, the changes to occur at 0100 UTC. So this year UK will revert to UTC on 28 October.