Quintilis means 'fifth' just as 'quartile', which comes from quartilis would refer to a 'fourth' -- as in the '25th percentile'. Since Quinitilis refers to a month, it should refer to the fifth month of the year, or May, but it doesn't. The calendar year of the Romans originally began in March, so the counting begins with March. The fifth month is therefore July, which used to be called Quintilis until it was renamed July to honor Julius Caesar. Likewise, August was Sextilis or 'sixth' until it was renamed to honor Augustus. September is based on the Latin for 7; October, for 8; November, for 9; and December, for 10.