What was special about the solar calendar of the Egyptians?

The Egyptians developed the world's first solar calendar. They concluded that the year had 365 days. It was based on two things; first the average period between two successive floods in the River Nile (which was 365 days) and second, the appearance of the star, Sirius in the sky after 365 days. They then, divided the year into 12 months with each month having 30 days. Five days were added for religious festivals.