The Holocene Songfacts reports that the song title comes from both a Portland bar where Justin Vernon had a "long dark night of the soul" and the current climactic era, which began some 12,000 years ago and follows on from the last major Ice Age.
Vernon was born and brought up in Eau Claire and still resides there. The song has three verses about three different times if his life living in the Wisconsin town. He…
\"Someway, baby, it's part of me, apart from me\"
You're laying waste to Halloween
You fucked it, friend, it's on its head, it struck the street
You're in Milwaukee, off your feet