Autumn includes two of my favourite days of the year: the autumn equinox, on or around the 22nd of September, and Oiche Samhain (Hallowe'en) October 31st. At the equinoxes the sun rises exactly due east and sets exactly due west, and day and night are of equal duration. I like equinoxes because of the feeling of the year being balanced and also because they are a pivot point for change: in the autumn the brighter, warmer half of the year giving way to the shorter, colder winter days; in spring, the sense of emerging from the dark half into warmth and long days.
I moved to the Burren on the day of the autumn equinox in 1995. I rented a cottage covered in roses from a slightly strange, slightly intimidating but completely fascinating woman. Exactly ten years later I married that woman on the autumn equinox. At least it ensures that I'm unlikely to forget our wedding anniversary!
Equinoxes and solstices (shortest/longest days) were important in ancient Ireland. The 5000 year old passage grave at Newgrange (a UNESCO World Heritage Site) is aligned so that its inner chamber is illuminated by the rising sun at the winter solstice. Nearby Knowth has two passages, aligned to the rising and setting sun at the equinoxes. In a time before calendars (in the sense we have them) these astronomical events would been important indicators of changing seasons and hence of times to plant and harvest. The first full moon after the autumn equinox is still referred to as the Harvest Moon, its light providing a much needed extension to the working day during harvest time.
Oiche Samhain (the night of Samhain) was one of the two most important nights of the Celtic year, which was divided into a dark half (beginning at Samhain) and a light half (beginning at Beltane, May 1st). These are the modern dates, originally the feast days would likely have been either the cross-quarter day (more anon) or the full moon closest to it. If you think of the equinoxes as dividing the year into two equal halves, and the solstices dividing the halves into quarters, then the mid-points between each equinox and the following solstice divide the year into eighths and these are called cross-quarter days. There are Christian holy days that now correspond roughly with these dates and there is a school of thought that maintains that this is not accidental: that is, the dates were chosen to both build upon and suppress the earlier pagan festivals, in a way similar to that in which early churches were often built on existing sacred sites.
In Irish tradition, at Samhain the gateways between the two worlds were open: our world and the underground world of the Tuath de Danaan. These were (or are) the fairy people who live now in the numerous tombs and mounds throughout Ireland, having been driven underground after losing a great battle with the Milesians. It is thought to be dangerous to be out and about after dark at Samhain in case one accidentally travels to the other world and is unable (or simply forgets) to return. The other world is Tir na nOg (the land of the young) a country of great beauty, fruitfulness and perpetual youth, perhaps not such a bad place to get lost in. The dwellings of the Tuath de Danaan, raths (ring forts) and mounds, often have hawthorn trees growing on them, trees sacred to the fairies. It is said to be deeply unlucky to cut down one of these trees, worse still to damage a rath or mound. Such is in the strength of this tradition today that a new motorway being built to bypass the town of Ennis had to be diverted to avoid the necessity of cutting down a particularly important fairy tree. Keep an eye out for it when driving north from Shannon airport.