One practice which continued into the Christian period in Ireland was the observation of the quarter days. These were agricultural milestones that divided the year into four. They are Imbolc (1 Feb), Beltane (1 May), Lughnasadh (1 August), and Samhain (1 November). Each of these was associated with a different point in the agricultural year. Imbolc was the beginning of spring and was when ewes started giving milk. Beltane was the start of summer and was when cows were moved to the summer pastures. Lughnasadh was the beginning of autumn and the harvest. Samhain was the beginning of winter and the end of the harvest.
In pre-Christian times, Beltane and Samhain were the most important of the four feasts. More survives about how they were observed than the other two. As for the evidence of Christianity adapting pre-Christian practice, I'll go through them one at a time. As usual with my posts about medieval British and Irish holidays, I'm drawing heavily here on Ronald Hutton's The Seasons of the Sun: A History of the Ritual Year in Britain.
Imbolc (1 February)
We have no written evidence of how Imbolc was celebrated in pre-Christian times. We know only that from an etymological standpoint it does seem to have referred to milking. However, there has been plenty of speculation about the pre-Christian rites associated with this day because of its Christian renaming as the feast of St Brigid. St Brigid was the patron saint of Kildare and the most important Irish saint after Patrick. However, scholars generally debate whether the saint was a historical person, or whether she was a Christian reworking of the Irish goddess Brigit for whom Brigid of Kildare was named.
Even if the saint was a historical person, elements of the goddess's worship could still have been incorporated into the celebration of the saint's feast day. Adding further confusion to the mix is the fact that the pan-Christian feast of Candlemas was also celebrated on St Brigid's Day. While the eternal flame burning at Kildare has often been speculated to be a pagan survival, thanks to the name Brigit meaning "fiery arrow", flames are the whole point of Candlemas, with candlelit processions being part of the celebration in Britain and Ireland by the 7th century, which is also the earliest attestation of the cult of St Brigid. Another complication is that the goddess Brigit is never associated in texts with flames or agriculture, the two main features of the saint's cult, but is instead mainly a patron of the arts and sometimes a battle goddess for Leinster.
There are many folkloric practices associated with St Brigid's Day, recorded in writing from the 18th century - over a thousand years after the Christianisation of Ireland. The most famous of these is the weaving of a St Brigid's Cross. While some have tried to argue that this might be a pre-Christian ritual, it's a bit tricky to say that about a Christian cross. Sometimes beds of rushes and small offerings of food were also put out for the saint, and an effigy of her was made in straw, both of which could possibly relate to an earlier practice for Imbolc, but no one can say for sure. Some of the old customs associated with St Brigid's Day are closely tied to ideas of virginity and youth, which could apply equally to a virgin nun as to a pagan celebration of spring fertility.
Beltane (1 May)
Beltane's pre-Christian rites are much better-attested than Imbolc's. Early medieval Irish writers were Christians, but they noted that the feast had been presided over by druids in pre-Christian times. The 10th century glossary traditionally attributed to Cormac of Cashel says of Beltane: "lucky fires, i.e. two fires which Druids used to make with great incantations, and they used to bring the cattle against of the diseases of each year to those fires." Although Beltane was not elided with any major Christian feast, it continued to be marked as a major turning point in the agricultural year with many folkloric rituals. Sending the cows off to the summer pastures, usually to be minded by young women at temporary housing called shielings, remained a major part of life for Gaelic-speaking peoples in Ireland and Scotland into the 20th century. Dairying had been happening at summer sheilings since the early medieval period.
Accounts of Irish folk customs from the 19th century show that leading cows over and through fires was still an important part of Beltane, nearly a thousand years after the glossary had recorded it as an ancient practice. The flames of the Beltane fires were thought to protect the cows' milk from fairies. People also leapt over the fire to seek good luck and protection, often accompanied by Christian prayers. Elaborate local rituals varied from place to place. While some of them were clearly later Christian folk elaborations, such as using rowan flowers to keep away witches, it's obvious that the core elements of Beltane were pre-Christian in origin. It's perhaps the best example of pagan survival in Christian Ireland. The holiday was not turned into a Christian holiday at all. It was simply left alone by the Church, while the Christians who continued to celebrate it infused it with Christian devotional activities themselves, which were layered on top of the underlying pagan elements.
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