Halloween, October 31, "hallowed" or "holy evening." More properly, All Hallows Eve, the day before All Hallows Day on November 1, and All Saints Day on November 2. Thus today is the first of a trio of Christian holy days which were once recognized collectively as Hallowmas or Hallowtide, three successive days of feast and celebration—holy days of obligation—and the time in the liturgical year dedicated to honoring the dead…recognized saints (hallows), martyrs, and departed believers.
Of course our modern Halloween bears little resemblance to this ancient observance, having during its many-centuries-long metamorphosis picked up traditions and influences from various sources—particularly those found throughout Scotland and Ireland—especially encompassing aspects of the old Celtic harvest festivals and the Gaelic Samhain. Over the last half-century the media—Hollywood, the movies and T.V.—have so reinvented and transformed Halloween that there's scarcely a single original recognizable aspect remaining.
Trick or treat.