ancient pagan Irish holiday.
Samhain in particular... fall is when the harvest ends and the earth symbolically 'dies'.... it's when animals are slaughtered for the upcoming cold season.... it's natural that it became associated with death.
Christianity simply harnassed those natural ideas and renamed them and directed them toward christian aims...
One of the reasons Christianity is the largest religion on the planet is that it's easily the most adaptable among the major religions.
Neither Islam or Judaism or Buddhism or Hinduism or Confucianism has been able to incoporate other religious traditions as well.