A week is a human convention
There are seven days in a week because we inherited a seven-day cycle shaped by ancient astronomical and religious traditions. A week is not forced on us by one complete turn or orbit in the way that a day or year is.
You can watch Earth turn daylight into night. Finding a physical event that marks every Monday is a harder task. The week belongs to the way people have organised time.
Seven familiar lights in the sky
Royal Museums Greenwich connects the tradition with ancient Babylonian astronomy and with the seven-day religious cycle in Judaism. The history has several strands, rather than one securely identified inventor.
The ancient sky-watchers counted seven important moving lights: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Calling them the classical planets describes an old grouping. Today we classify the Sun as a star and the Moon as Earth’s natural satellite.
Why four weeks are not a lunar month
Four seven-day weeks make 28 days. The Moon takes about 29.5 days to return to the same phase, as NASA explains in its guide to Moon phases. The two cycles are close enough to invite a connection, but they are not equal.
A calendar month is not always four weeks either. March has 31 days: four complete weeks and three more days. The weekday cycle keeps going when a new month begins.
The extra day at the end of the arithmetic
A common Gregorian year contains 365 days. Since 52 × 7 = 364, that is 52 complete weeks plus one day. A 366-day leap year has two days left over.
Those spare days help explain why a date moves through the weekdays from year to year. For the other units in the story, revisit days, months and years, or follow the Moon’s changing appearance.