All you wanted to know about Samhain, but were afraid to ask - thanks to Dictionary of Celtic Lore, James McKillop (Oxford 1996).
Jim Carroll

Samain, Samhain, Samhuinn (ScG), Sauin (Manx) [cf. Olr. sam, summer; fain, end]. Irish, Scottish Gaelic, and Manx names for the seasonal feast of pre-Christian origin fixed at 1 November on the Gregorian calendar. The most important of the four great calendar feasts of Celtic tradition, in¬cluding, by their old Irish names, *Beltaine (1 May), *Imbolc (1 February), and Lugnasad (Modlr. Lunasa/*Lughnasa, 1 August); its counterparts are in Wales *Hollantide, in Cornwall *Allantide, and in Brittany Kala-Goanv. The antiquity of Samain is attested to by the Coligny Calendar (1st cent, BC) which cites the feast of Samonios. The same source explains that to the ancient Gauls the period of dark precedes the light, supporting the commonly held belief that Samain is the equivalent of New Year's Day. Julius * Caesar (ist cent, BC) reported that the Gaulish *Dis Pater, god of death and winter's cold, was especially worshipped at this time of year. Other * classical commentators observed that *Teutates might be worshipped at this time by having sacrificial victims drowned in vats, whereas sacrifices to *Taranis were burned in wooden vessels. Samain's equivalents on the Christian cal¬endar are All Saints' Day (introduced by Pope Boniface IV in the 7th cent, to supplant the pagan festival of the dead) and Halloween.
By abundant testimony, Samain was the principal calendar feast of early Ireland. Each of the five provinces sent assemblies to *Tara for a *feis held every third year. At *Tlachtga the lighting of the winter fires was a key part of the Samain ceremony. In part Samain ceremonies commemorated the *Dagda's ritual intercourse with three divinities, the *M6rrigan, *Boand, and *Indech's unnamed daughter. Just how much of this remembrance included *fertility rites, or what their nature might be, is not known; but in Irish and Scottish Gaelic oral tradition, Samain time was thought most favourable for a woman to become pregnant. At *Mag Slecht in Co. Cavan, human sacrifices might be offered to *Crom Cruaich, called the 'chief idol of Ireland' by early Christian scribes. Although the full nature of Crom Cruaich is not known, popular writers on early Ireland have taken to calling him Samain, implying that he gave his name to the seasonal feast; although at least one American encyclopaedia repeats this conjecture, it is un¬supported by early Irish texts.
Authors of early texts are careful to point out when important action takes place at Samain. At this time the predatory *Fomorians would exact their tribute of grain, milk, and live children. Each year on this date * Aillen mac Midgna came to burn *Tara until *Fionn mac Cumhaill dispatched him. From *Cruachain in Co. Roscommon came the triple-headed monster *Aillen Trechenn who wreaked havoc on all of Ireland, especially *Emain Macha and Tara, until he was eliminated by *Amairgin (1). *Cuchulainn encountered Other¬worldly damsels at Samain time, and this was also the time *Caer and * Angus 6g flew off in *swan form.
The different celebrations of Samain over the centuries explain some of the traditions still popu¬larly attached to Halloween. Standing between the two halves of the Celtic year, Samain seemed sus¬pended in time, when the borders between the natural and the supernatural dissolve and the spirits from the *Otherworld might move freely into the realm of mortals. Concurrently, humans might perceive more of the realm of the dead at this time, and looked for portents of the future in games. People might choose from small cakes called barmbracks [Ir. bairin breac, speckled loaf, i.e. with currants or raisins] containing a ring or a nut to determine who would be married and who would live singly. Bonfires were built in parts of Ireland and Gaelic Scotland. It. was also a time to relax after the most demanding farm work was done. In counties Waterford and Cork, country lads visited farmers' houses on the night before Samain, oiche shamhna [Samain eve], collecting pence and provisions for the celebrations. In Cork the pro¬cession of young men blowing horns and making other noises was led by someone calling himself the White Mare, wearing white robes and the con¬figuration of a horse's head. On the Isle of Lewis in the Outer Hebrides, though the inhabitants were Protestant, people gathered *ale and other provisions for a mock ceremony, calling Shoney of the sea to enrich their grounds in the coming year. Turnips were hollowed out with candles put inside.