The ancient civilization credited with creating the first known calendar is the Sumerians, who lived in Mesopotamia around 3000 BCE. They developed a lunar calendar based on the cycles of the moon, consisting of 12 months. This calendar was significant not only for agricultural planning, allowing Sumerians to predict seasonal changes, but also for religious events and rituals. Their innovations laid the groundwork for later calendars, including the Babylonian and, subsequently, the Julian and Gregorian calendars used today. The Sumerian calendar reflects the importance of astronomy in their culture and demonstrates early humanity's efforts to organize time orderly.