Today – 22nd September 2020 – is the first day of Autumn. Today is the Autumn equinox, when day and night are of roughly equal length. The nights will now become increasingly longer than the days, until the Spring equinox when the pattern is reversed. It also marks the time of year when the northern hemisphere begins to tilt away from the Sun, resulting in less direct sunlight and consequently the cooling temperatures.
When it is asked ‘When does Summer end and Autumn begin’ meteorologists base their answer on the beginning of astronomical Autumn which is defined by the Earth’s axis and orbit around the Sun. So this year Autumn begins today on 22 September 2020 and ends on 21 December 2020.