There is no precise date that we know of for when he wrote it. He was a prolific writer and his poems often reflected the times and places in which he lived. However, it is likely that the poem was written sometime between the 1930s and the 1950s, as it reflects a more traditional, pre-war view of Christmas that would have been familiar to Betjeman's generation.
It is worth noting that the poem was first published in 1954, in the anthology *The New Oxford Book of English Verse*. However, that does not necessarily indicate the exact date of composition.