More commonly known as Halloween, the ancient festival of Samhain marks the end of the harvest season, with aspects of a festival of the dead. It became associated with the Catholic All Saints' Day in the ninth century; there's quite a lot of additional cultural baggage associated with it. While it was frequently observed in Ireland and Scotland for many centuries, it began to grow to its current prominence in the USA in the 19th century, as a response to lingering puritan prejudice against other festivals; when people want to party, if you ban one excuse they'll find another. (Which, incidentally, is why Hogmanay — new year's eve — is such a big deal in Scotland, where once the Kirk banned Christmas festivities as ungodly).
But what I find most interesting about it is that it recurs every 365 days (give or take the odd leap year).
We humans are slaves to the calendar. From our origins as plains-dwelling scavengers through the early millennia of agriculture, our diet was dominated by the climate, and the climate in turn depended on the cycle of the seasons almost everywhere. The cycle of seasons is hugely significant if you're a hunter (some prey species disappear at certain times of year; others are subject to behavioural variation (for example, mating seasons). It's important if you're a gatherer of wild fruit and seeds, and it's a matter of life or death if you're a subsistence farmer scratching away at the soil to raise one harvest a year. Hunter-gatherers can, at least in principle, move to follow the food supply; farmers are on the wrong side of a Malthusian one-way door, their population density at least an order of magnitude higher. Running away to find a less over-grazed region isn't an option on the menu. And so we observe the calendar, marking off the significant end-points of the season with an almost superstitious dread. All Hallows Night marks the end of the harvest season and allows us to rub shoulders with the dead; and if it hasn't been a good harvest, we might be joining them before Beltane.
For a thousand generations this has been our lot. But these days we live in a time of plenty — an age when "seasonal vegetables" are optional, bananas cost less than potatoes in western European supermarkets, and intensive farming can deliver two or three harvests in a single year (and more, when greenhouses and hydroponics are used).
And what does it mean to live somewhere utterly divorced from the Earth's seasonal cycle?
I may be sceptical about space colonization, but I'm an optimist about exploration. I'm fairly certain that at some point in the next one to two decades human beings will visit the moon again — this time for an extended duration stay, spending long months and possibly years in conditions as unfriendly as those any 19th century arctic explorer could have imagined. In two or three decades, there'll probably be a base camp on Mars; not a city, not a colony, but nevertheless a habitat of sorts (and one that will probably be occupied for years — the cost of going to Mars is so prohibitive that in this day and age a 48-hour excursion to raise the flag looks positively feckless).
What are their festivals and cycles going to be, those explorers who visit worlds where the sidereal year is something other than 31,558,149.540 seconds?