This word means "the twentieth day of the month" (e.g., October 20th) or "twenty days" when referring to a duration. It is an irregular reading used for specific dates and durations in Japanese, similar to 十日.

月(つき)の20(はつか)日(か)目(め)、または20(はつか)日(か)間(かん)という期(き)間(かん)。日(ひ)付(づけ)や日(にっ)数(すう)を数(かぞ)える特(とく)別(べつ)な読(よ)み方(かた)です。