tricennal

The story

1. Latin · derived fromtrīcennālis“tricennial”
2. Medieval Latin · derived fromtrīcennāle
3. English todaytricennal

From mistaken use of Medieval Latin trīcennāle, neuter substantive of Latin trīcennālis (“tricennial”), from trīcennium (“30-year period”) + -ālis (“-al: forming adjectives”), from trīcennis (“30-year”) + -ium (“-ium: forming abstract nouns”), from trīciēs (“30 times”) + annus (“year”) + -is (“forming compound adjectives”). Doublet of tricennial.