Short for 'bissextile' or 'bissextus,' relating to leap year; the extra day added to February in leap years.
From Latin 'bis' (twice) and 'sextus' (sixth), referring to the repeated sixth day before the March calends in the Roman calendar. The term evolved from Julian calendar calculations.
The Romans called leap day 'bissextus'—literally the 'second sixth day'—because they counted backward from March and counted the sixth day twice. Modern 'bisext' comes straight from this ancient Roman calendar system.
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