Today is ##Halloween, the best day of the year. It's a time for eating sweets, dressing up, and going wild on the decorations. Self-expressions is fundamentally baked into this most wonderful of days.
You'll often hear it referred to as #Samhain, but this is incorrect. Samhain actually falls on 1 November, and Hallowe'en is Samhain Eve. Just as Nollaig (Christmas) is preceeded by Oíche Nollaig, so Samhain is prcedded by #OícheShamhna (Halloweeen)
Well, more or less.
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#Samhain is an old #Celtic festival marking the beginning of winter, which appears to have been the Celtic New Year. We know very little about how the pagan Celts celebrated Samhain, since they were suspicious of this new-fangled "writing" idea, and the #Romans weren't too interested in recording the customs of non-civilised people. All we really have is a reference to "Samonios", which is how the Romans represented a continental Celtic term.
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#AllSaintsDay, or #AllHallowsDay, is commonly described as a #Christian coöption of Samhain, but that's also not the case. When All Hallows' Day was introduced, it was celebrated on a range of days, including 1 November. In #Ireland and #Scotland, however, it was originally celebrated on 23 April. So if it was intended to replace a #pagan festival, that festival was #Bealtaine, not Samhain.
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Samhain also didn't fall on 31 October or 1 November. It fell on the halfway point between the southward equinox and the southern solstice, which by the Gregorian calendar would be around 23 October.
All Hallows' Day eventually moved to 1 November across Christendom to make things more consistent, but this was after all the Celts had converted to Christianity.
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That said, All Hallows' Day was always a much bigger deal in Ireland and Scotland than in other places, and Ireland and Scotland are where old Celtic culture held out the longest. Since All Hallows' Day falls next to Samhain, the significance of Samhain got transplanted on to All Hallows' Day.
Something similar may have happened in Mexico, with the #Aztec #DayOfTheDead mapping on to All Hallows' Day and sort of intersecting with #Halloween.
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Trick or treating, surprisingly, has a mostly Christian origin. Charity was encouraged on All Hallows' Day, and it took the specific form of handing out soul cakes to the poor. But since soul cakes are nice, people who weren't necessarily destitute would wear costumes to try and wrangle some out of well-meaning neighbours. It became part of the game to pretend not to recognise people in disguise, and that evolved into just wearing costumes.
https://www.youtube.com/watch?v=tpWdzXJObZ0
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Much of the lore around #Halloween really derives from 19th century historians looking for #pagan survivals in the modern day. Druids didn't go door to door demanding gifts and threatening horrors if the house owners didn't comply; that was just a bit of fun started by Christians in17th century.
But there is some (vague) evidence that the pagan Celtis did think of Samhain as a liminal time, when the sídhe could enter the world of the living. That all combines to give us modern #Halloween
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