It's the time of year when, every year, I get annoyed by the Christians at one extreme and Neo Pagans at other extreme posting silly things about the ancestry of Halloween and the ancient Celtic festival of Samain. I thought I'd try to forestall some of the less factual assertions by a list of bullet points about what we actually know about Samain and the Celts, since Samain is one of the most popular Neo Pagan festivals or feast-days.
In 609 Pope Boniface IV designated November 1 All Saints' Day. The night before, October 31 consequently became All-Hallows Eve, or Halloween. In A.D. 1000, the church made November 2 All Souls' Day, a day to honor the dead. There are references to both feast days earlier in some places, but these are the dates of official sanction.
Most authorities seem to agree that there is some relationship, at least in terms of an inspiration, for All Souls Day, between the Celtic feast of Samain and All Souls.
Samain is the Old Irish spelling; in Modern Irish it's Samhain, in Scottish Gaelic Samhuinn, in Manx Sauin, and Gaulish Samonios. Samain and Samhain are both pronounced like the modern English noun sow followed by -in; SOW-in. The first syllable is emphasized.
There are assertions that Samain derives etymologically from Old Irish sam "summer" + fuin "end." This is not likely linguistically speaking; it is, however, a successful folk etymology, and one that dates to the medieval Irish glosses.
We do not really know what the druids did, or the Celts, in terms of rituals associated with Samain. We know it was important enough to be marked on the late 1st century B. C. E. Coligny calendar (Cunliffe 1997, 188), and that it gave its name to the eleventh month of the year and an autumn feast, Samonios.
Despite the popularity of the assertion, there is no medieval or earlier indication that Samain was perceived, or celebrated, at all, as the "Celtic New Year" with the festive implications that New Year has in modern cultures. The Coligny divides the year into a dark half and a light half. We also have a reference from Julius Caesar in De Bello Gallico 6:18 that the Celts "define the space of time not by the number of days but of nights."
It does seem from the Calendar, and references in Medieval Irish texts, that Samain was perceived as a dividing point between the light half of the year, and the dark half, and that, as the divider, Samain was a liminal time between times—and thus in some ways, a time outside of time.
Samain marks the end of the summer grazing, and the herds and flocks being moved from summer pasture, and the beasts that aren't going to be bred being butchered. That final harvest of fall crops, and communal butchering, offered a time for feasting as well, as food that won't keep is consumed.
Although outside of time, Samain is not timeless. The Coligny Calendar notes the trinox Samoni, the three nights of Samain. Various medieval Irish texts associate Samain as a feasting period where people gathered for feasts, and decisions about law, and paying tribute in grain and milk.
The medieval Irish tale of Serglige Con Culainn, The Love-Longing of Cu Chulainn/The Wasting-Sickness of Cuchulain/The Sick-Bed of Cuchulain opens with the people of Ulster assembled at Mag Muirthemni for the annual November 1 festival of Samain:
Oenach dognithe la Ultu cecha bliadna .i. tri la ria samfuin 7 tri laa iarma 7 lathe na samn feisne. Iss ed eret no bitis Ulaid insin i mMaig Murthemni, oc ferthain oenaig na samna cecha bliadna. Ocus ni rabe isin bith ni dognethe in n-eret sin leu acht cluchi 7 cheti 7 anius 7 aibinnius 7 longad 7 tomailt, conid de sin atat na trenae samna sechnon na hErend.
Each year the Ulstermen held a fair; the three days before Samain and three days after it and the day of Samain itself. That is the time that the Ulstermen used to be in Mag Muirthemni holding the fair , and nothing was done by them during that time but games and gatherings and pleasure and eating and feasting, so that it is from that come the thirds of Samain throughout Ireland.
Perhaps the most Samain-appropriate tale of all, particularly given the associations we have with Halloween, is the Ectra Nera, The Otherworld Adventure of Nera. It's set at Samain, and has otherworld adventures, and talking severed heads and all sorts of autumnally appropriate reverberations.