Today is the Autumnal Equinox - the half way point between the longest day of the year on the Summer Solstice (June 21) and the shortest day of the year on the Winter Solstice (December 21). Equinox comes from Latin for "equal night" because day and night are each about 12 hours in length on this date (in summer, days are about 15 hours long and in winter they're about 9 hours long at our mid-latitude location in NY).
The reason for the change in seasons is not what most people immediately think when asked. It's not due to the changing distance from the Earth to the Sun (while the Earth's orbit is elliptical, it's not by much and the difference is only about 3%). As a matter of fact we're closest to the Sun in early January.
The reason for seasons, and why we can mark solstices and equinoxes, is because the Earth's axis is tilted by about 23.5°. In the Northern Hemisphere summer, we're tilted toward the Sun (and the Southern Hemisphere is tilted away and has winter) and vice versa six months later. On the Summer Solstice, when we have maximum tilt toward the Sun, the Sun is directly over 23.5° N latitude - the Tropic of Cancer. On the Winter Solstice, when we have maximum tilt away from the Sun, the Sun is directly over 23.5° S latitude - the Tropic of Capricorn. On the equinox, like today, the Sun is directly over the Earth's Equator.
In modern Wicca, the Autunmnal Equinox is called Mabon after a Welsh diety. Mabon is sometimes also known as the Feast of the Ingathering and is based on a traditional harvest festival. Many ancient cultures knew about and certainly celebrated the equinox in some way. The Mayan city of Dzibilchaltun, built around 200 AD (some argue for an earlier date), has a temple called the Temple of the Seven Dolls. Every equinox, the Sun shines through a passageway in the temple. It was intentional.
Halfway around the world at the Loughcrew megalithic cairn T in Ireland, some 5,000 years old, the equinox sunrise illuminates a passage and an engraved panel at the end.
There are numerous other archeoastronomy sites around the world which show similar alignments. It's only natural to mark the passage into the "dark" part of the year.