The Gaulish calendar appears to have 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 month Giamonios (the April/May lunation). The entire year may have been considered as beginning with the 'dark' half, so that the beginning of Samonios may be considered the Celtic New Year's Day. The celebration of New Year itself may have taken place during the 'three nights of Samonios' (Gaulish trinux[tion] samo[nii]), the beginning of the lunar cycle which fell nearest to the midpoint between the autumnal equinox and the winter solstice. The lunations marking the middle of each half-year may also have been marked by specific festivals. The Coligny calendar marks the mid-summer moon (see Lughnasadh), but omits the mid-winter one (see Imbolc). The seasons are not oriented at the solar year, viz. solstice and equinox, so the mid-summer festival would fall considerably later than summer solstice, around 1 August (Lughnasadh). It appears that the calendar was designed to align the lunations with the agricultural cycle of vegetation, and that the exact astrological position of the Sun at that time was considered less important.
In medieval Ireland, Samhain became the principal festival, celebrated with a great assembly at the royal court in Tara, lasting for three days. After being ritually started on the Hill of Tlachtga, a bonfire was set alight on the Hill of Tara, which served as a beacon, signaling to people gathered atop hills all across Ireland to light their ritual bonfires. The custom has survived to some extent, and recent years have seen a resurgence in participation in the festival.
Samhain was identified in Celtic literature as the beginning of the Celtic year and its description as "Celtic New Year" was popularised in 18th century literature. From this usage in the Romanticist Celtic Revival, Samhain is still popularly regarded as the "Celtic New Year" in the contemporary Celtic cultures, both in the Six Celtic Nations and the diaspora. For instance, the contemporary calendars produced by the Celtic League begin and end at Samhain.
The Samhain celebrations have survived in several guises as a festival dedicated to the harvest and the dead. In Ireland and Scotland, the Féile na Marbh, the 'festival of the dead' took place on Samhain.
The night of Samhain, in Irish, Oíche Shamhna and Scots Gaelic, Oidhche Shamhna, is one of the principal festivals of the Celtic calendar, and falls on the October 31. 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. It is still the custom in some areas to set a place for the dead at the Samhain feast, and to tell tales of the ancestors on that night.
Traditionally, Samhain was time to take stock of the herds and grain supplies, and decide which animals would need to be slaughtered in order for the people and livestock to survive the winter. This custom is still observed by many who farm and raise livestock because it is when meat will keep since the freeze has come and also since summer grass is gone and free foraging is no longer possible.
Bonfires played a large part in the festivities celebrated down through the last several centuries, and up through the present day in some rural areas of the Celtic nations and the diaspora. Villagers were said to have cast the bones of the slaughtered cattle upon the flames. In the pre-Christian Gaelic world, cattle were the primary unit of currency and the center of agricultural and pastoral life. Samhain was the traditional time for slaughter, for preparing stores of meat and grain to last through the coming winter.
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. Often two bonfires would be built side by side, and the people would walk between the fires as a ritual of purification. Sometimes the cattle and other livestock would be driven between the fires, as well.
Gaelic custom of wearing costumes and masks, was an attempt to copy the evil spirits or placate them. In Scotland the dead were impersonated by young men with masked, veiled or blackened faces, dressed in white. Candle lanterns (Gaelic: samhnag), carved from turnips were part of the traditional festival. Large turnips were hollowed out, carved with faces, placed in windows to ward off evil spirits.
Guisers — men in disguise, were prevalent in 16th century in the Scottish countryside. Children going door to door "guising" (or "Galoshin" on the south bank of the lower Clyde) in costumes and masks carrying turnip lanterns, offering entertainment of various sorts in return for food or coins, was traditional in 19th century, and continued well into 20th century. At the time of mass transatlantic Irish and Scottish immigration that popularized Halloween in North America, Halloween in Ireland and Scotland had a strong tradition of guising and pranks.
Divination is a common folkloric practice that has also survived in rural areas. The most common uses were to determine the identity of one's future spouse, the location of one's future home, and how many children a person might have. Seasonal foods such as apples and nuts were often employed in these rituals. Apples were peeled, the peel tossed over the shoulder, and its shape examined to see if it formed the first letter of the future spouse's name. Nuts were roasted on the hearth and their movements interpreted - if the nuts stayed together, so would the couple. Egg whites were dropped in a glass of water, and the shapes foretold the number of future children. Children would also chase crows and divine some of these things from how many birds appeared or the direction the birds flew.