Called the Guidizzolo inscription, this rustic Roman calendar, found in norther Italy (near Verona), in the province of Gallia Cisalpina, mentions this feast as
'XV Kalendas Ianuarius Eponae'.
This date--counting backwards fifteen days from the Kalends of January (i.e. January 1), would lead to today's date, December 18th, as explained here:
--American journal of archaeology, Volume 8
Of course, why Epona was worshipped in December is unknown; but it is interesting to note the old Welsh custom of the Mari Lwyd held around the time of the new year (i.e. January 1), wherein a mumming troop carry around a horse's skull on a pole, the jaws hinged so that the skull can talk and engage in rhyming contests, earning food for the mummers. The origin of the custom is unknown, but certainly Wales had its own horse goddess in Rhiannon.
And so, have a happy Eponalia!
For more about Epona, check out Epona.net, the most comprehensive site on the subject.
1. The other two are the famous "TRINOX[tion] SAMO[nii] SINDIV" i.e. "three-nights of Samonios today" on the 17th of Samonios; when the month of Samonios occurred is of much debate; the other is "DECAMNOCTIACIS GRANNI", the ten-night festival of Grannos, about which we know nothing, not even when it occurred.