Among the pagan Celts of Ireland, the most important festivals seem to have been associated with the solar crossdays, halfway between the equinoxes and solstices. Imbolc, in early February, is the Celtic equivalent of Groundhog Day (today!); Bealtaine is the Celtic Mayday; Lughnasa, near the first of August, is the crossday that has mostly been forgotten, although not in the west of Ireland (it came to international prominence lately in the title of an award-winning play by the Donegal playwright Brian Friel, Dancing at Lughnasa); and Samain, in early November, corresponds to our Halloween, All Saints Day and All Souls Day. `
Little is known of how Imbolc was observed before the coming of Christianity. Days grew noticeably longer, winter's back was broken, and people suffering from claustrophobia and sun deprivation could certainly have used some cheering up. The day is now the feast of Saint Bridgit, an early Irish saint whose story is richly embroidered with pagan associations carried by the Celts on their long migration from Central Europe to the Atlantic fringe.
More of all of this in my latest book, Climbing Brandon: Science and Faith on Ireland's Holy Mountain.