The calendar is “Gregorian based,” but also reflects Biblical days.  Biblical days start at night, so the ‘khodesh’ (renewed crescent of the moon) begins in the evening (Erev) of the Gregorian day on which it is posted.  “High Days” are listed on the day they start [at night, because there is most always an in-home blessing to set apart that ‘day’], AND the event of the ‘assembly’ on the day and at the time the assembly actually occurs.