HALLOWEEN.
The Druids were ancient Celtic priests [and priestesses] who shared the Buddhists' belief in rebirth and the indestructibility of the mind. They regarded the seasons of the year as being a metaphor for the death and rebirth of the human being. Halloween represented the death of the old year and was believed to be the time of year when the veil separating the human and ghost [PRETA] realms was at its thinnest.
Yule (the winter solstice) was the time of conception of the coming year and Imbolc (Candlemas) was the actual birth of the New Year, with the appearance of the first lambs and green shoots. The period between Yule and Candlemas was the gestational period when the new animal and plant life, though growing and stirring, was still hidden in the body of its mother, or in the case of vegetation within the body of Mother Earth.
The significance of Halloween to Buddhists now becomes clear. In the Druid system the period of seven weeks between Halloween and Yule is the gap between death of the old and conception of the new year. This corresponds to the 49 days of the "bardo "[temporary limbo after death in Tibetan Buddhism].
Halloween thus symbolizes the entry of the disembodied consciousness [a continuing process, not an entity] into the intermediate state between leaving one [physical] body and occupying another. In traditional [Tibetan and Mahayana] Buddhist beliefs the "bardo"-consciousness will experience hideous apparitions -- ghosts, demons, and so on.
If the mind reacts with panic then a samsaric rebirth, possibly in unpleasant realms, is inevitable. However, if the "bardo"-being recognizes these apparitions as hallucinations -- projections and reflections of its own negative karma resulting from [unskillful] actions -- then liberation remains possible.
The reasons for the Druidic custom of dressing up as ghosts, demons, and so on may be to symbolize that these scary "bardo" apparitions are in fact nothing other than aspects or appearances of the person's own self.
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Francis Barrett - The Magus Celestial IntelligencerRowan Moonstone - Pagan History The Origins Of Halloween