Samhain is the first and, in some ways, most important of the great Celtic feasts. It is remembered throughout the Celtic territories as the end and the beginning of the year's cycle, and a time when the gates between the worlds stand open. In the agricultural year, Samhain is the season during which herds of cattle were culled. A few were kept for breeding stock while many were butchered to sustain the clans through the winter. Thus the season between Samhain and midwinter was a time of feasting and plenty.
The change from the light to dark half of the year meant that the powers of the spirit world were especially active. Common folk considered the night of Samhain a time to keep to their halls and seek the protection of the gods and goddesses as they enjoyed the bounty of the passing year. Some tales tell of raiders from the otherworld who every year burn the hall of Erin's king.
Of course the feast of Samhain was Christianized as the feast of All Saints and All Souls by the church in the Middle Ages. It is commonly assumed that this reflects a Pagan custom of honoring the ancestors and heroes at Samhain that fits well with the agricultural meaning of the season, and is universally accepted in modern Paganism. Mythically there are several patterns proper to Samhain. One key tale has the Dagda mating with the Morrigan, their loving creating a fertile river. Another refers to Donn, the first Ancestor of Erin, who gathers the year's dead ones and on Samhain sends them to the Isles of Summer in the West.
All these strands combine in the modern Celtic Neopagan holy day of Samhain. It is celebrated as Harvest Home, when the year's labor is ended and we enjoy the fruits of our effort. It is the feast of the dead, when we honor our ancestors, the heroes, and those who have passed in the past year. It is the gate time, when works of magic and spirit-contact are especially strong. It is New Year's night, when the old passes away and the new begins in darkness.