A week corresponds by convention to seven days. One year is made up of 52 weeks plus 1 day (for the common year of 365 days) or plus 2 days in case of leap year.
These additional days, originating the 53th week, are considered belonging to the new year, when the first week has at least four days belonging to the new year (from Thursday to Sunday). As example the week, that includes 29th December on Monday, is the first week of the year. On the contrary if 1st January falls on Friday, Saturday or Sunday, this day belong to 52th or 53th week of the previous year.
The term week derives from the old German word “wik” meaning change, succession series, turn.
The seven days week was widespread by the Greeks as taken from the Middle East civilizations, as the Chaldeans, the Mesopotamians and Egyptians. According to their beliefs the planets influence that day assigned under their rule. Chaldeans names of the days were translated into Greek and then in Latin in the way we know.
dies Lunae − Monday (eng) Montag (ger) from the moon goddess, Mona or Mana
dies Martis − Tuesday (eng) Dienstag (ger) from the god Týr, assimilated to Mars
dies Mercurii − Wednesday (eng) from the god Woden or Odin (ger Mittwoch means midweek)
dies Jovis − Thursday (eng) Donnerstag (ger) from the god Thor, god of thunder, like Jupiter
dies Veneris − Friday (eng) Freitag (ger) from the goddess Freyja, assimilated to Venus
dies Saturni − Saturday (eng) from the Latin god Saturn (ger Samstag derives from Sabbatum)
dies Solis − Sunday (eng) Sonntag (Ger) from the Roman Dies Solis
The introduction of the seven days week took place in Rome by decree of Emperor Constantine in 321 AD. The Romans have adopted until then a cycle of 9 days within the month, the Nundinum. Following the Calends, after 5 or 7 days depending on the length of the month, were the Nones, corresponding to the first quarter moon phase. Then came the Ides on 13th or 15th day of the month, i.e. full moon.
Ides is a Latin word deriving from the Etruscan iduare, which means to divide, as the full moon is just dividing a month in two parts.
By the time the Christian faith became the majority in the Roman Empire, they replaced the Latin names of the last two days of the week, as sacred days. Dies Saturni was called Sabbatum or Sabbata, translation of the Jewish Sabbath, the day dedicated to God for the Jews, as the first Christian were of Jewish origin.
Constantine's decree ratified that the sacred day was no longer Saturday but Sunday, called by the Emperor “dies Solis”, referring to the eastern cults widespread in the empire, like that of Mithras.
According to the Gospel Christ’s resurrection was on the third day after Friday, so the feast day dedicated to God became the dies Solis. Then the Christians in Latin area called the Sunday “Domini dies”, day of the Lord, and later forbade the celebration on Saturday. The other days remained dedicated to ancient pagan gods whether Greek, Latin, Germanic or Slavic according to the countries.
In the Dark Ages these days were considered with disapproval: in the Catholic liturgy, the days are numbered starting from Sunday as first day, second day and so on. In the modern calendar in Portugal this use has been kept: after the Sunday, Monday is the second day and is called the segunda-feira and so until Friday, that is the sexta-feira before Sabado, Saturday.
Image: "Calendar of Saints". A medieval manuscript fragment of Finnish origin, about 1340-1360 AD, used by the Dominican convent of Turku, Finland.
Source: Wikimedia Commons