Short answers, pulled from the story.
King Sargon of Akkad issued a decree acknowledging the astrological significance of a seven-day period around 2300 BCE. This decree emerged from the ancient Near East and marked the beginning of the seven-day week tradition. The Babylonians later inherited this tradition and venerated the number seven because it corresponded to the seven celestial bodies visible to the naked eye.
Emperor Constantine the Great decreed a seven-day week throughout the Roman Empire on the 3rd of March 321. This imperial edict made Sunday, known as the Day of the Sun, a legal public holiday and effectively ended the coexistence of the older eight-day nundinal cycle. The transition from the eight-day week to the seven-day week was not immediate, as both systems coexisted for a while after the Julian calendar came into effect in 45 BCE.
The names of the days of the week in English and many other languages derive from a sophisticated planetary hours system that connected each day to a specific celestial body and its associated deity. This naming convention resulted in succeeding days being named for planets that are three places apart in their traditional listing, a characteristic discussed in Plutarch's treatise written around 100 AD. The system created a heptagram within the circle of planets, forming a {7/3} heptagram that connected the days in a specific sequence that has remained remarkably stable across cultures and centuries.
Muslims observe their day of congregation, known as Jumu'ah, on Friday because it was described as a sacred day of congregational worship in the Quran. According to Islamic beliefs, the seven-day week concept started with the creation of the universe by Allah, with Muhammad stating that Allah created Adam after Asr on Friday. The Islamic creation narrative mirrors the seven-day structure while maintaining Friday as the day of congregational worship, creating a unique religious calendar that has influenced billions of people across the Middle East and beyond.
The International Organization for Standardization uses Monday as the first day of the week in its ISO week date system through the international ISO 8601 standard. This system is adopted by most of Europe and China, while North America, South Asia, and many Catholic and Protestant countries consider Sunday the first day of the week. The ISO week date system dictates that each week begins on a Monday and is associated with the year that contains that week's Thursday, ensuring that week 1 of any year can be determined by specific rules involving the day of the week on the 1st of January.
The Soviet Union implemented five and six-day work weeks between 1929 and 1940, with peak use of the five-day work week occurring on the 1st of January 1931 at 72% of industrial workers. From summer 1931 until 1940, each Gregorian month was subdivided into five six-day work weeks. These alternative systems demonstrate that the seven-day week is not the only possible way to organize time, yet none have achieved the global dominance of the seven-day cycle.