bissextile

From Latin bisextilis/bissextilis (annus) (“bissextile year”), from bisextus + -ilis, from bis- (“two; twice; doubled”) + sextus (“sixth”) + dies (“day”), from the Julian calendar's original reckoning of its quadrennial intercalary day as a 48-hour February 24, subsequently distinguished as the two separate days of the sixth day before the March calends (ante diem sextum Kalendas Martias) and the "doubled sixth day". (February 24 is now normally understood as five days before the first of March, but was called the sixth by the Romans owing to their inclusive counting of dates. See Roman calendar on Wikipedia.)

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