One Christmas was so much like another, in those years around the sea-town corner now and out of all sound except the distant speaking of the voices I sometimes hear a moment before sleep, that I can never remember whether it snowed for six days and six nights when I was twelve or whether it snowed for twelve days and twelve nights when I was six.
I don't suppose I'm the first person who, on reading the opening lines of Dylan Thomas's A Child's Christmas in Wales, has been inspired to check the Met. Office records. At any rate, I have joined their company, and can report that the Christmases of 1920 (when Thomas was six) and 1926 (when he was twelve) were both notable for the lack of snow in the Swansea area.
In December 1920, "abnormally mild weather [...] set in on the 18th and continued until the end of the year." The report adds that "rain was general on Boxing Day" - a nice echo of Joyce's "snow was general all over Ireland", I think. December 1926 too was unusual, this time for being "the driest December in the past 43 years": only about a quarter of average precipitation for the month fell in England and Wales. This is not a promising setting for six days and nights of continuous snow at the Thomas household.
So, full marks for an atmospheric evocation, Dylan, but bottom of the class for meteorology. This is why you should never write poetry without a licence.