What calendar concept did Ancient Greece develop that included a 13th month every three years?

The lunisolar calendar is the correct choice because it is a calendar system that incorporates elements of both lunar phases and the solar year. Ancient Greece utilized the lunisolar calendar, which accounted for the discrepancies between the lunar cycle and the solar year by adding a 13th month, known as an intercalary month, every three years. This adjustment helped to synchronize the calendar months with the seasons, ensuring that agricultural and religious events aligned properly with the solar year.

In contrast, the solar calendar solely focuses on the solar year and does not adjust for lunar cycles, while the Julian and Gregorian calendars are variations of solar calendars that do not incorporate a periodic intercalary month. The Julian calendar, established by Julius Caesar, was a reform of the Roman calendar that improved its accuracy by introducing a leap year every four years but did not include a 13th month. The Gregorian calendar, introduced later, made further refinements to the Julian calendar but also did not include this feature. Thus, the lunisolar calendar concept reflects the combination of lunar and solar elements that Ancient Greece developed to maintain their calendar system.