Egyptians most accurate calendar was based on?

The Egyptians' most accurate calendar was based on the lunar cycles and the annual flooding of the Nile River. They developed a civil calendar consisting of 365 days, divided into 12 months of 30 days each, with an additional five epagomenal days. This calendar was crucial for agricultural planning and religious festivals. The heliacal rising of the star Sirius also played a significant role in marking the New Year and the flooding season.