Calendar

In Judaism, Saturday is still observed as the Sabbath and the last day of the week.

The convention of grouping days into seven-day weeks originated in ancient Babylonia. The scriptures of Abrahamic religions, which arose in the same region, also describe God creating the world in six days and resting on the seventh.

By ISO standard, the week starts on Monday. "Weekend" (週末) literally means "week's end," placing Saturday and Sunday last. Yet many countries start on Sunday—because in 321 CE, Constantine I made it a rest day, and "work then rest" felt more natural.

The International Fixed Calendar, proposed by British accountant Moses Cotsworth in 1902, divides the year into 13 months of exactly 28 days each. Every date always falls on the same day of the week, so you would never need a new calendar—the same single page works for life.

Year 0 does not exist in the AD calendar. The year before AD 1 is 1 BC. Because of this, the 21st century began not in 2000 but in 2001.