The Companion Guide to Wales describes Cwm Gwaun as one of the most important meltwater channels from the last Ice Age in the entire British Isles. The River Gwaun rises in the Preseli Mountains and carved this valley out as the glaciers retreated, leaving steep wooded sides and a long, narrow floor. Tributaries cut side-valleys into the slopes. The trees that grew back are the classic Welsh upland mix - sessile oak, beech, alder, rowan, ash, willow - a forest that climate, geology, and grazing have together shaped over ten millennia. The Rough Guide to Wales calls Cwm Gwaun one of the great surprises of Pembrokeshire, a bucolic vale of impossibly narrow lanes surrounded by the bleak shoulders of bare mountains. The community covers 3,870 hectares. There are 21 listed buildings inside it, two Grade II listed bridges at Llanychaer and Picton Mill, a Grade II listed medieval parish church dedicated to St Brynach, and a Baptist chapel called Jabes with one of the few surviving outdoor baptistries in Wales - a basin filled directly from the river.

The Hen Galan celebration begins the night before, on 12 January, with gathering at the Dyffryn Arms - known locally as Bessie's. The pub is Grade II listed, has been run by Bessie Davies's family since 1845, and was originally called Llwyn Celyn, Holly Bush. In 2015 it appeared in the Good Beer Guide for the fortieth consecutive year. A serious fire damaged the pub in February 2019, but it reopened in June that year. The next day is the festival proper. Children from the local primary school - Ysgol Llanychllwydog - are absented to take part. They walk from house to house through the valley, singing traditional Welsh-language songs. In return the householders give them calennig: traditionally food to help through the winter, now sweets and small amounts of money. A horse skull mounted on a pole and draped in a white sheet, the Mari Llwyd, appears at doorways and tries to gain entry through verse contests. The household sings back, refuses entry, and offers food when defeated. Anyone who refused to welcome and reward the visitors was said to receive a llond y ty o fwg - a house full of smoke - meaning a year of bad luck.

The Dyffryn Arms is not the only place in the valley to drink. The Gwaun Valley Brewery, a small craft operation at Kilkiffeth Farm, makes beer that finds its way back to Bessie's bar - a closed local loop of grain, hops, and consumption. A small hydroelectric scheme at Pontfaen generates enough power for about seventy homes, drawing water from the same River Gwaun that the chapel uses for baptisms. The cumulative effect is a valley that runs partly on its own energy, drinks its own beer, sings its own songs, and keeps its own time. Pontfaen, the community's main hamlet, sits four miles southeast of Fishguard - close enough to reach the sea in twenty minutes by car, far enough up the valley that mobile signal is patchy and Welsh-language radio works better than English. The primary school operates entirely in Welsh.

Calendar reforms tend to win, because most communities have no particular reason to refuse them. So why did Cwm Gwaun refuse? The valley is steep, wooded, hard to reach by the old roads, and culturally distinct - a pocket of Welsh-language Pembrokeshire surrounded by anglicised lowland. The Church in Wales kept the Gregorian calendar; the local nonconformist tradition, with its strong Welsh-language identity and instinctive resistance to official change, did not particularly insist. The Dyffryn Arms gave the community a fixed social point at which to celebrate, year after year, on whatever date the old reckoning pointed to. None of this would have mattered without the children, whose participation gave the tradition its annual renewal. Today the BBC sends reporters most January. National newspapers run a feature. The valley shrugs. The calendar reform was, from Cwm Gwaun's point of view, somebody else's idea - and 273 years later, it is still somebody else's idea.