The months September, October, November, and December derive from Latin words that mean “seven,” “eight,” “nine,” and “ten” respectively. So why are they applied to the ninth, tenth, eleventh, and twelfth months of the year? The answer lies in the messy history of marking the year, described in detail in David Duncan’s book, Calendar: Humanity’s Epic Struggle to Determine a True and Accurate Year. This is part of a complete episode.