On the Celtics Wheel of the Year, there are 8 seasonal festivals; Yule, Imbolc, Ostara, Beltane, Litha, Lughnasadh, Mabon and finally, Samhain. Samhain marks the end of the harvest season and the beginning of the darkened times—the decent into winter, and the longest and coldest nights if the year.
Much like Beltane, Samhain is considered to be a time where there is a thinning of the veils between worlds. This is why it's believed that spirits, faeries, ancestors, and darker entities could more easily roam our lands, as the portal between our world and theirs is considered to be at its thinnest. Samhain is a time of newness and beginning—with Samhain considered to be the start of the Celtic New Year. This may seem strange to many who see new beginnings and fresh starts as this bright and energized passage, but it's only through the portal of death that we find our rebirth. And that is what Samhain represents—our descent into death. This is not a time to bound forward in outward journey and achievement, but a time where we are urged to go inward—honoring the darkness that cloaks this night. As one of the most famous Celtic festivals (thanks to its modern ancestor, Halloween), Samhain is filled with rich history and old tradition. It is a time where we honor the dead, protect the living, and petition the spirits for wisdom to help guide us.