We grew up planning our lives, our schedules, according to the seven days of the week. For most of us, Monday through Friday are hectic days, then Friday night seems like a party night, and then Saturday and Sunday come and the lazy weekend begins. But have you ever wondered why this cycle repeats itself for exactly 7 days? Why not 6 or 8? Why does our weekly routine consist of only 7 days, who established these rules, why and how the week was created? It takes approximately 24 hours (23 hours and 56 minutes, to be exact) for the Earth to make a complete revolution around its axis, and that is the length of a day. In addition, we make one revolution around the Sun in about 365 days (again, 365 days, 5 hours, 59 minutes and 16 seconds, to be exact), and this corresponds to our perception of the year. Almost all of our perceptions of various measures have an astronomical meaning associated with them. Therefore, there is obvious reason to believe that the concept of a week of 7 days must also have some kind of astronomical connection. The origin of the concept of 7 days in a week Cosmic phenomena have always guided humanity in different ways. In ancient times, some civilizations were avid contemplators of the sky, and one such civilization was the Babylonians, an ancient society whose people lived in Mesopotamia (modern Iraq). Observing the cosmic wonders, the Babylonians invented their original calendar, which demonstrated the movement of the moon. The calendar was also supposed to show the transition of the moon from one phase to another: from the full to the waning half, to the new half, and finally to the rising half. Apart from this, there are several other stories to explain the length of the 7-day week. The Babylonians were not only curious astronomers, but they were also known for their knowledge of astrology. The Babylonians are said to have drawn up a horoscope in which each day of the week corresponded to one of the classical planets. The classical planets are the seven celestial bodies visible to the naked eye. These include the Sun, Moon, Mars, Mercury, Jupiter, Venus and Saturn. Of course, calling the Sun and Moon planets is not correct. The Babylonians referred to each of the 7 days as the 7 planets visible to the naked eye, which made the number 7 important in several ways. Moreover, the seven-day structure of the week is thought to have some connection to the Book of Genesis in the Hebrew and Christian Bible, according to which God created the world in seven days, where six days of work were followed by one day of rest. Among these common conceptions, however, the most common is that of the phases of the moon. Other Structures of the Week Although the idea of a week with 7 days is almost ubiquitous these days, the number 7 was not the only number associated with the number of days in the week in history. The Babylonians adhered to a 7-day structure. There were other civilizations for which the definition of the week was different. For example, the Egyptians had their own Egyptian calendar in which the week was 10 days, and the Romans had an 8-day week according to the Roman calendar. But the Babylonians were such a culturally dominant part of society that gradually their idea of the week spread around the world. Today almost every continent, every country, every city, town, and village has switched to a seven-day week. Time after time, attempts have been made to change the concept of a seven-day week, but it seems that the seven-day week is here to stay for a long time.