Contrary to what we learned as children, the day marking the end of summer and welcoming autumn is not always 21 September. Astronomically speaking, the autumnal equinox, meaning the day when summer ends in the northern hemisphere and there are the same number of light and dark hours, can fall between 21 and 24 September. The 2022 autumnal equinox will be on 23 September, at 3:04 am Italian time to be precise. Why does this date vary? The Earth takes 365 days and six hours to carry out a complete revolution around the Sun (solar year). This difference of six hours is compensated with the introduction of leap years: every four years, a day is added to the calendar. This variation also determines the changing dates of the equinoxes and solstices. The autumnal equinox, in particular, most often falls on 22 or 23 September, while the 21st and 24th are rather rare. It suffices to say that the last time the equinox fell on 21 September was over 1,000 years ago, while the last equinox on 24 September was in 1931 (the next one will be in 2303). Lovers of tradition can instead rejoice, because in this century the autumnal equinox will fall on 21 September twice, in 2092 and 2096.