A week is made up of seven days. Monday, Tuesday, Wednesday, Thursday, and Friday are weekdays, whereas Saturday and Sunday are weekends. The concept of a seven-day week dates back to Babylonian times.
The whole cycle of the moon is around 28 days, according to Babylonian astronomers. The cycle was separated into four phases: new moon, waxing half moon, full moon, and waning half moon. One week is represented by each phase, which consists of seven days. In the year 321 A.D., the Roman Emperor Constantine made the seven-day week an official feature of the Julian calendar. During the first millennium A.D., China, Japan, and India developed the seven-day week as well.
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