Short answers, pulled from the story.
A week is a unit of time equal to exactly seven days, or 168 hours, 10,080 minutes, or 604,800 seconds. There are just over 52 weeks in a Gregorian calendar year, specifically 52.1775 weeks in a mean Gregorian year of 365.2425 days.
The origins are debated, but the continuous seven-day week was first practiced in Judaism, dated to the 6th century BC at the latest. The Babylonians used holy days on the 7th, 14th, 21st, and 28th of their lunar month, but their week was not continuous. Emperor Constantine the Great officially decreed the seven-day week in the Roman Empire in AD 321.
The days were named for the seven classical planets, which in the ancient world included the Sun and Moon alongside five visible planets. The Germanic peoples matched each Roman planetary deity with a native equivalent, giving English Sunday (Sun), Monday (Moon), Tuesday (Tiw/Mars), Wednesday (Woden/Mercury), Thursday (Thor/Jupiter), and Friday (Frige/Venus). Saturday kept the Roman name for Saturn because no Germanic equivalent existed.
Emperor Constantine the Great officially decreed the seven-day week in the Roman Empire in AD 321 and on that occasion made Sunday a public holiday. Before that decree, the seven-day week had been widely known throughout the empire since at least the 1st century AD, coexisting with the older Roman eight-day nundinal cycle.
Revolutionary France replaced it with a ten-day week called the decade, used from October 1793 to April 1802. The Soviet Union shifted most factory workers to five-day and six-day work cycles between 1929 and 1940, with the five-day system covering 72% of industrial workers at its peak on the 1st of October 1930. Neither system permanently displaced the seven-day week.
Jews observe the Sabbath on Saturday, the seventh day, in honor of God resting after six days of creation. Christians observe Sunday, the first day in Christian calendars, in honor of the resurrection of Jesus. Muslims observe Friday as the sacred day of congregational worship, described in the Quran as set apart for that purpose.
ISO 8601 defines each week as beginning on Monday and assigns it to the year that contains that week's Thursday. Week 1 of any year is the week containing the first Thursday of January. Under this system, the first week of a year may begin in late December of the previous year.