The Roman festival of Eponalia is celebrated on 18th December
Here is a reference to her worship from The Golden Ass by Lucius Apuleius (second century c.e.):
"These thoughts were interrupted by my catching sight of a statue of the goddess Epona seated in a small shrine centrally placed, where a pillar supported the roof-beams in the middle of a stable. The statue had been devotedly garlanded with freshly picked roses. So in an ecstasy of hope on identifying this assurance of salvation, I stretched out my forelegs and with all the strength I could muster, I rose energetically on my hind legs. "