Imbolc, also called Brigid's Day, is a Gaelic traditional festival marking the beginning of spring. It is held on 1 February, or about halfway between the winter solstice and the spring equinox. Historically, it was widely observed throughout Ireland, Scotland and the Isle of Man
The Romans were turned back and never conquered the Celtic lands of Ireland, Scotland, the Isle of Man, Wales, and Cornwall on the British Isles, and Brittany in what's now Western France. Their traditions were never influenced by the Romans, they jumped straight to Christianity.
This holiday has ancient Celtic roots. Brigid was a goddess in Celtic Ireland, and her feast day was Feb 1 - which marked the beginning of Spring. She seems to be the Irish version of the Dawn Goddess, associated with spring, fertility, healing, and so on. Likely not a coincidence the early Christians chose to name the early Spring holiday for their St Brigid.