The Sumerians and Babylonians were probably the first people to use what we now recognise as a modern calendar. The basis of this was the lunar cycle and the Sumerian year was made up of 12 lunar cycles. But 12 moon cycles fall short in relation to the solar year. So not to fall out of sync with the seasons, the Sumerian astronomers introduced an extra month every four years. The high time for Sumerian astronomy and astrology was the 6th century B.C. and a group of scribes, among them Enuma Anu Enlil were at the forefront. It is still unclear how these scribes were trained and where all this knowledge came from. But what is certain is that most of them worked later at the Babylonian court where they helped to shape the Babylonian calendar. Compared to the later famous Babylonian astronomy, these were indeed humble beginnings, but they laid the foundation of the houses, star signs and the zodiac. It was also the beginning of astrology, when aspects in the sky and movements of the planets were observed and recorded and then certain symbolic meaning derived from repeat events.
During this period, the first day of each month (beginning at sunset) continued to be the day when a new crescent moon was first sighted—the calendar never used a specified number of days in any month and finishes again with the last sighting of the Waning Moon. Then follows 2 days of New Moon period when no moon is visible.
Subsequently the Sumerian calendar was not only absorbed into the Babylonian calendar, but a lot of other cultures such as the Ancient Greeks, Egyptians and Hebrews also absorbed elements into their own calendar system. In particular the Sumerian calendar was used as a blueprint of many religious calendars who are still in use today.
From the 5th century B.C. the Sumerian calendar slowly transformed into a luni-solar calendar, meaning that both the solar and the lunar cycles played an important part and were brought into alignment. This principle was later formally described by the Greek astronomer Methon of Athens around 430 B.C. He concluded that 19 solar years equal exactly 235 lunar months and this discovery that underpins the luni-solar calendar is now called the Metonic cycle.
The Living With the Moon lifestyle calendar is also based on the Ancient Lunisolar Calendar, now believed much older than the Babylonian Calendar. Some sources put its existance back to the Minoan civilisation (2700-1450 B.C).