Because the holiday we now call Halloween was originally the eve of the first day of winter, as evident in the names I listed above. This, of course, is why the winter solstice is called Midwinter--because it fell between the beginning of winter on November 1st, and the end of winter on February 1, called Imbolc in Irish and Candlemas in English (and surviving in the US as Groundhog's Day, which has its own tradition of predicting the coming of spring).
Halloween is the beginning of winter. We're far enough from the equinox to notice that the nights are getting longer, the weather is cold, the mornings have frost. It's not hard to see how this transition, this slide into the dark time of the year brought us associations with death; the plants are dying, the harvest is reaped, the animals are slaughtered* to be cured and eaten over the winter. It's only natural that death is on the mind, and from that closeness with our dying environment, we draw closer to our own departed. Both the Dead and Winter are integral parts of Halloween; in fact, I'd say they're inseparable.
So it's with no small amount of amusement and annoyance** that I woke up to see it snowing on October 29th. It's really unusual for us to get snow in October here in the Philly suburbs; I mean, we've occasionally gotten a few flakes, but so far we've got at least three inches, probably four, and it's not going to stop snowing until tonight. We're not ready for this kind of weather--none of us were. Our decorations are getting blasted by the wind and snow; I hope that the rubber bats and spider don't disappear on us.
So my husband and I are spending the day watching Halloween cartoons, eating the food for a cancelled party, and drinking, and occasionally going out to shovel the walk.
We're getting the real meaning of this old Celtic holiday--winter isn't coming, it's already here, kids.
Consider this Nature's Trick; the treat is... um... I guess it looks nice?
*In fact, in Welsh the month of November is called Tachwedd meaning "slaughter"; compare with the Anglo-Saxon name for November, Blōtmōnaþ, "blood month".
**Annoyance, as it has resulted in me having to postpone my Halloween party for a week. At which point it's not a Halloween party, is it? I guess we'll call it a Guy Fawkes Party, or something.