The names of the days of the week have ancient origins that stretch back through history. It all began with the Sumerians, who established the concept of time in months, based on the moon's full cycle around the Earth. The Babylonians took this concept and expanded it into weeks, with seven days each. But why seven days? The Babylonians noticed that seven days approximated the time between different moon phases and matched the number of visible planets in the sky known to humans at the time: the Moon, Mars, Mercury, Jupiter, Venus, Saturn, and the Sun.