It is biennial.
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Biennial -sg that happens every alternate year.
Biennial means "occurring every two years," OR "lasting for two years." The word biannual can also mean "occurring every two years," although its most common meaning is "occurring twice a year."
Annal
The adjective is quadrennial. (extension of 1-annual, 2-biennial, 3-triennial) (The archaic word for a five-year period is a lustrum.)
Year 1000 was a medieval word. It was approximately in the middle of the Middle Ages. It starts with the letter Y.