The habit of a midwinter festivity had come by the dawn of history (and probably very long before) to seem a natural one to the British, and not one to be eradicated by changes of political or religious fashion.... It was general custom in pagan Europe to decorate spaces with greenery and flowers for festivals, attested wherever records have survived.
Stations of the Sun: A History of the Ritual Year in Britain (ed. OUP Oxford, 2001) - ISBN: 9780191578427