The Sumerians created the first calendar by recording the movements of the moon and stars on clay tablets stored in libraries. They realized a lunar year was too short, so added an extra month every 3-4 years called a "leap month". The calendar was important to Mesopotamians as it helped them know when floods would occur and when to plant crops, as well as paying taxes and attending festivals. The uses of the calendar, such as paying taxes and attending festivals, were crucial as they let people know important upcoming dates.