Hallowed Evening
"The Sun is setting and the day is beginning" is what you might hear the ancient Celts say. Likewise, they would acclaim when the last harvest is done, at the end of what we would currently call October, and when the constellation Pleiades sets in the sky, it is the end of the year and the beginning of the next cycle.
The pastoral and agricultural peoples of old felt the primal, subltle surges of nature's next movement, below, deep and dark, within the earth's womb. Starting dormant in the Fall and continuing it's secret, underworld maturation during Winter, only to spill forth a wonderful sprouting bounty come next Spring. This seemed right.
The storage of crops for the darker, colder Winter months, mirrored nature's striving toward a resting period, to recharge and restore. Early Celtic people created the holiday Samhain (pronounced sow-in or sow-een, depending on what part of the country you were from). This was the most important holiday of the year, also called Little Sun, Last Harvest or Summer's End. This is a time of transformation and quiet inner searching.
The Samhain holiday begins sundown, October 31st, but the effects last the whole month of November, during which the veil between worlds becomes thin and there is more spirit activity than any other time of the year. It is known as the Festival of the Dead, when the departed walk with the living and hoards of faeries lead an entouage of the dead across the threshold; a sometimes ruckus and wild ride across the land.
At the end of harvest, feasts and offerings of food, sweets and drink, are given with respect to keep the hungry guests happy and satisfied and to gain their favor in the coming year. This is politely acknowledged in modern day with children and adults alike donning costumes and roaming the land collecting or giving candy. "Trick or Treat", hence, the slightly trivial greeting.