Samhain
(from the Old Irish samain) is the word for November in the Gaelic languages. The Scottish Gaelic spelling is Samhainn or Samhuinn (for the feast), or an t-Samhain (for the month). The Festival of Samhain is a celebration of the end of the harvest season in Gaelic culture, and is generally regarded as 'The Celtic New Year'.
The same word was used for a month in the ancient Celtic calendar, in particular the first three nights of this month, with the festival marking the end of the summer season and the end of the harvest. A modernized version of this festival continues today in some of the traditions of the Catholic All Souls' Day, the secular Halloween, and in folk practices of Samhain itself in the Celtic Nations and the Irish and Scottish diasporas. It is also observed by various types of Neopagans.
Neopaganism
Samhain is observed by various Neopagans in various ways. As forms of Neopaganism can differ widely in both their origins and practices, these representations can vary considerably despite the shared name. Some Neopagans have elaborate rituals to honor the dead, and the deities who are associated with the dead in their particular culture or tradition. Some celebrate in a manner as close as possible to how the Ancient Celts and Living Celtic cultures have maintained the traditions, while others observe the holiday with rituals culled from numerous other unrelated sources, Celtic culture being only one of the sources used.
Wicca
Samhain is one of the eight annual holidays, often referred to as 'Sabbats', observed as part of the Wiccan Wheel of the Year. It is considered by most Wiccans to be the most important of the four 'greater Sabbats'. It is generally observed on October 31st in the Northern Hemisphere, starting at sundown. Samhain is considered by most Wiccans as a celebration of death and of the dead, and it often involves paying respect to ancestors, family members, elders of the faith, friends, pets and other loved ones who have died. In some rituals the spirits of the departed are invited to attend the festivities. It is seen as a festival of darkness and death, which is balanced at the opposite point of the wheel by the spring festival of Beltane, which Wiccans celebrate as a festival of life and fertility.
While the Wiccan version of Samhain is not a form of reconstruction, and is largely mixed with other traditions in a form of universalism, it is influenced by the Celtic holiday from which the name was taken.