Samhain is the word for November in Irish, and in Scottish Gaelic is spelt Samhuinn. 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. Elements of the festival are continued in the traditions of All Souls Day and Halloween. The name is also used for one of the sabbats in the Neo-Pagan wheel of the year.
Irish samhain is from Old Irish samain, samuin, samfuin, referring to 1 November (lathe na samna, "samhain day"), and the festival and royal assembly at that date in medieval Ireland (oenaig na samna, "samhain night"). Its meaning is glossed as "summer's end", and the frequent spelling with f suggests analysis by popular etymology as sam "summer" and fuin "sunset, end". Old Irish sam "summer" is from PIE *semo- , cognates are Welsh haf, Breton hañv, Old Norse language sumar all meaning "summer", and Sanskrit sáma "season".
W. Stokes in KZ 40:245 (1907) suggests an etymology from Proto-Celtic *samani with a meaning "assembly", cognate to Sanskrit sámana, Gothic samana). Compare to this cetemain "1 May, beltane", related to Middle Welsh kyntefin "1 May, calan haf, May" from *kintu-samino- "beginning of summer" (G. Murphy in Early Irish Lyrics 52), mehefin "June, middle of summer". J. Vendryes in Lexique Étymologique de l'Irlandais Ancien (1959) concludes that these words containing *semo- "summer" are unrelated to samain, remarking that furthermore the Celtic "end of summer" was in July, not November, as evidenced by Welsh gorffennaf "July".
We would therefore be dealing with an Insular Celtic word for "assembly", *samani or *samoni, and a word for "summer", saminos derived from *samo- "summer" (alongside samrad *samo-roto-). Irish samain would be etymologically unrelated to "summer", and derive from "assembly". But note that the name of the month is of Proto-Celtic age, c.f. Gaulish SAMON[IOS] from the Coligny calendar, and the association with "summer" by popular etymology may therefore in principle date to even pre-Insular Celtic times.
Confusingly, Gaulish Samonios (October/November lunation) corresponds to GIAMONIOS, the seventh month (the April/May lunation) and the beginning of the summer season. Giamonios, the beginning of the summer season, is clearly related to the word for winter, PIE *g'hei-men- (Latin hiems, Slavic zima, Greek kheimon, Hittite gimmanza), c. f. Old Irish gem-adaig "winter's night" (the vocalism of gam "winter" is influenced by sam, Thurneysen KZ 61:253). It appears, therefore, that for some reason already in Proto-Celtic the first month of the summer season was named "wintry", and the first month of the winter half-year "summery", possibly by ellipsis, "[month at the end] of summer/winter", so that samfuin would be a restitution of the original meaning after all. This interpretation would either invalidate the "assembly" explanation given above, or push back the time of the re-interpretation by popular etymology to very early times indeed.
Bealtaine, Lúnasa and Samhain are still today the names of the months of May, August and November in the Irish language. Similarly, Lùnasdal and Samhain are the modern Scots Gaelic names for August and November.
The Celtic calendar divided the year into two halves, the "dark" half, beginning with the month Samonios (the October/November lunation), and the "light half", beginning with the Giamonios (the April/May lunation). The entire year appears to have been considered as beginning with the "dark" half, so that the beginning of Samonios may be considered the Celtic New Year's day. All months began at full moon, and the celebration of New Year took place during the "three nights of Samonios" (Gaulish trinux[tion] samo[nii]), the full moon of nearest 1st November. Likewise, the beginning of the summer season was celebrated at the full moon nearest 1st May (see Beltane). The full moons marking the middle of each half-year may also have been specific festivals, the Coligny calendar marks the mid-summer one (see Lughnasadh), but omits the mid-winter one (see Imbolc). Note that the seasons are not oriented at the solar year, viz. solstice and equinox, but that the mid-summer festival would be considerable later than summer solstice, around 1 August. It appears that the calendar was designed to align the lunations with the agricultural cycle of vegetation, and that the actual movements of the Sun were less important.
In medieval Ireland, samain remained the principal festival, celebrated with a great assembly at the royal court in Tara, lasting for three days, consistent with the Gaulish testimony.
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.
In parts of western Brittany Samhain is still heralded by the baking of kornigou, cakes baked in the shape of antlers to commemorate the god of winter shedding his "cuckold" horns as he returns to his kingdom in the Otherworld.
The Romans identified Samhain with their own feast of the dead, the Lemuria. This, however, was observed in the days leading up to May 13.
With Christianization, the festival in November (not the Roman festival in May) became All Hallows' Day on November 1st followed by All Souls' Day, on November 2nd, after which the night of October 31 was called All Hallow's Eve, and the remnants festival dedicated to the dead eventually morphed into the secular holiday known as Halloween.
In some types of neopaganism, particularly those influenced by Wicca, Samhain is one of the eight solar holidays or sabbats. It is celebrated in the northern hemisphere on October 31 or November 1 and in the southern hemisphere on May 1.
The holiday, with Beltane, is one of the most popular among Neopagans, and public Samhain rituals invariably attract large gatherings. It is the last of the harvest festivals (after Lammas and Mabon); in some traditions it symbolizes the death of the old god.
Among the sabbats, it is preceded by Mabon and followed by Yule.
From an astrological perspective, the setting of Pleiades, the winter stars, heralds the supremacy of night over day and the start of the dark half of the year that is ruled by the realms of the moon.