Mean Geimhridh, Ancient Celts
Mean Geimhridh (Irish Gaelic for "Celtic Midwinter") was a mysterious holiday celebrated by Proto-Celts and Druids, who lived in Ireland and the United Kingdom in the Bronze Age. If you want to get in on this one, we suggest building a complex monument that marks the solstice, like Stonehenge. No biggie, right? Pictured: Stonehenge in winter.