Why is the ninth month called September?
early 15c. alteration of Middle English parfit (c.1300), from Old French parfit "finished, completed, ready" (11c.), from Latin perfectus "completed, excellent, accomplished, exquisite," past participle of perficere "accomplish, finish, complete," from per- "completely" (see per) + facere "to perform" (see factitious). Often used in English as an intensive (perfect stranger, etc.).
"to bring to full development," late 14c., parfiten, from perfect (adj.). Related: Perfected; perfecting.