Samhain

Samhain (IPA: /ˈsawənʲ/) is the word for November in Irish; the Scottish Gaelic name Samhuinn is closely related. The same word was used for the first month of the ancient Celtic calendar, and in particular the first three nights of this month, the festival marking the beginning of the winter season. ...

Celtic folklore
The Samhain celebration survived in several guises as a festival dedicated to the dead. In Ireland and Scotland, the Féile na Marbh, the "festival of the dead" took place on Samhain.

Samhain Eve, in Irish and Scots Gaelic, Oidhche Shamhna, is one of the principal festivals of the Celtic calendar, and is thought to fall on or around the 31st of October. It represents the final harvest. In modern Ireland and Scotland, the name by which Halloween is known in the Gaelic language is still "Oíche/Oidhche Shamhna".

Bonfires played a large part in the festivities. Even into Christian times, villagers cast the bones of the slaughtered cattle upon the flames, cattle having a prominent place in the pre-Christian Gaelic world. The English word 'bonfire' derives from these "bone fires," but the Gaelic has no such parallel. With the bonfire ablaze, the villagers extinguished all other fires. Each family then solemnly lit its hearth from the common flame, thus bonding the families of the village together.

According to Irish mythology, during that night the great shield of Scathach was lowered, allowing the barriers between the worlds to fade and the forces of chaos to invade the realms of order, the material world conjoining with the world of the dead. At this time the spirits of the dead and those yet to be born walked amongst the living. The dead could return to the places where they had lived and food and entertainment were provided in their honour. In the three days preceding Samhain, the Sun God Lugh, maimed at Lughnassadh (August 1), dies by the hand of his Tánaiste (counterpart or heir), the Lord of Misrule. Lugh traverses the boundaries of the worlds on the first day of Samhain. His Tanist is a miser and, though shining brightly in the winter skies, he gives no warmth and does not temper the breath of the Crone, Cailleach Bheare, the north wind.

(Wikipedia)