Why do you think some early Cultures based their calendars on the cycles of the moon?

During ancient times, moon was considered sacred. It was the biggest thing visible to the human eye in the sky. The need of a calendar was important to keep a track of the crops growing in different seasons.

There the changing shaped of moon was observed and the placement of moon in the sky also changes with the changing seasons. Therefore, the changing shapes were considered as dates and the placement is considered as the season. It was first established by the Sumerians. Moon is still considered sacred.