It is called Samhain It is the beginning and end of the Celtic (and other Northern European Shaman based belief system) wheel of Sabbats.
It is, in effect, New Year. As it is a time of change, it was believed that this world and the spirit world were at their closest and that old friends could visit. The purpose of the scary masks and symbols was to frighten off evil, or simply lost spirits, and only to let those welcome through. It was adopted by the Church (who it must be said has a genius of adapting existing rituals and incorportating them into its calendar) and called it All Saints day - keeping the essential idea of remembering the dead.
As this is primarily a Northern hemisphere thing, it was also a period to take stock, slaughter live stock selected to provide food through the long winter months and basically batten down the hatches. Yule on the 21st of December, by contrast, was a celebration that the sun was unconquered and that Spring and a new harvest was on the way.
Samhain was very popular in Europe until probably the 18th century. It remained an annual event in rural Scotland (I recall in the early 60s making lanterns out of large turnips - pumpkins, it has to be said, are much easier to carve) and other less mainstream places and it has been re-introduced more broadly via the USA where early settlers took it and kept it alive.
In some ways the US for all its modernity is a bit like an ark for old European customs where settlers held on to old ways as a vestige of their culture whilst the original source moved on. An interesting example of cross re-fertilisation (if such a thing exists).
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