A week has seven days. Weekdays are Monday, Tuesday, Wednesday, Thursday and Friday, whereas weekends consist of Saturday and Sunday. The concept of a seven-day week originated in ancient Babylon.
Babylonian astronomers recognized that the full cycle of the moon consists of approximately 28 days. They divided the cycle into four distinct phases: new moon, waxing half, full moon and waning half. Each phase, which consists of seven days, represents one week. The Roman Emperor Constantine made the seven-day week an official aspect of the Julian calendar in 321 A.D. The seven-day week also developed in China, Japan and India during the first millennium A.D.