One of the mysteries of the English language finally explained.
The fourteenth day after a festival (this being the fifteenth day when counted inclusively, following ancient Roman practice); (hence) the period of fifteen days beginning with the day of a festival.
Late 15th century; earliest use found in Robert Fabyan (d. 1513), chronicler. From post-classical Latin quindena (also quintana) fifteenth day after a festival, counting inclusively, period of fifteen days from classical Latin quīndēnī fifteen each from quīndecim fifteen + -ēnī, suffix forming distributive adjectives.
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