Year of the Celt is a four-part series; each part covers a quarter of the Celtic year. Betrayals, violence, hope and love will touch the life of everyone before this year is through. Imbolc is the first book, beginning just after at the Celtic New Year (Samhain) and leading up to the night of the Imbolc festival. New love will offer hope to some, while the hidebound perish.
I looked into what people might have been eating then, the clothes they wore, their buildings, etc. A well-used source was a NASA website showing all the moon phases in the last 6,000 years. Many maps were referenced in order to find the location of various ancient settlements, hillforts, etc as well as plot credible routes to and from places. I would estimate that for every page I wrote I did at least 1 hour of research on top of the background knowledge.