Celtic Halloween called Samhain in Gaelic is a traditional Celtic festival marking the end of the harvest season. It also marks the beginning of winter also known as the "darker half" of the year because the days get dramatically shorter. Samhain is traditionally celebrated at sunset on October 31st and lasts until sunrise on November first. This is considered the halfway between the autumn equinox and the winter solstice. This time during Samhain was seen as a transitional time, when the vail between this world and the other side could be crossed without difficulty allowing Aos SÃ, or the 'spirits' like the 'fairies' into our world to create mischief. The Irish have many Celtic Halloween Traditions to ward off these spirits.
Celtic Anti-Fairy Measures for Halloween