The names of the days of the week in many languages are derived from the names of the classical planets in Hellenistic astrology, which were in turn named after contemporary deities, a system introduced in the Roman Empire during Late Antiquity.

In some other languages, the days are named after corresponding deities of the regional culture, either beginning with Sunday or with Monday.

In the international standard, Monday is now treated as the first day of the week.

Although this is the international standard, several countries, including the United States and Canada, consider Sunday as the start of the week.
The first day of the week in English is Sunday. Since ancient times it has been designated the "day of rest". The name of Sunday has something to do with ancient religions. The ancient Greeks knew that the sun was the source of life on the planet and they gave it prime importance in their thinking. When the Romans later adopted the seven day week they emphasize their respect for the sun by naming the first day of the week for it..."dies solis"..."day of the sun". The actual word "Sunday" is derived from the German word "Sonntag" (and they likely got it from the Scandinavians). Some tribes of these Germanic peoples invaded England in the 500's or so. They were known as the Angles and the Saxons. The old English work was "sunnandaeg" and it changed over time to become our current, "Sunday". There are several holidays that are traditionally held on Sunday. Mother's Day (second Sunday in May) and Father's Day (third Sunday in June) began to be celebrated rather recently, 1914 and 1924 respectively. Easter is an ancient holiday that was officially designated by the Church in the year 325. Its position on the calendar has confused lay people (laïques) for centuries. The date is calculated by taking the first Sunday after the first full moon on or after the 21st of March.
If sun is honoured on Sunday, it follows that the Moon should be given a place of prominence on Monday. And, indeed, the word for Monday is derived via the ancient Anglo-Saxon/Germanic tongue from the word for Moon, "monandaeg". Most other Western and Southern European languages use their own derivative of the word, "moon", as a root for "Monday". For example, the Latin word for moon is "luna" and from that comes the French word "Lundi". Monday has often been called a blue day. There is considerable speculation as to why this might be so. One theory runs that Monday (before the advent of the washing machine) was the usual day for washing and a blue dye (teinture, coloration) was frequently used to keep clothes from yellowing. Blue is also frequently associated with depression. Since Monday is typically the first workday of the week and workers must look over (attendre) a long depressing stretch of time until their next day-long break it has often been referred to as a blue day. Many cultures see Monday also as an unlucky day, even as a day when people become insane. This may have something to do with the day's association with the moon. The moon has been called inconstant, due to its tendency to wax and wane * (croître et décroître). The Latin word for moon comes even into our own language to designate lunacy. * To wax (in other context) : cirer, astiquer, farter…
Fat Tuesday is the designation generally given to the day just prior to the beginning of Lent (le carême). Lent is a 40 day time of fasting for many Christians and it always begins on a certain Wednesday prior to Easter. As the day before (always a Tuesday) is the last chance for revelry and also the last chance to use up perishables such as butter, lard and other fats that day has become known as "Mardi Gras" (French for Fat Tuesday). The word "Tuesday", however, was derived from a wholly different religious tradition. Tyr or Tiw was the Norse (Norvégien / Nordique / Scandinave) God of War. When the Germanic Angles and Saxon's invaded England in the 500's they supplanted a culture that had been heavily influenced by Rome for several hundred years. The day, Tuesday, had already been named for the Roman God of War, Martius (notice in French, Italian, and Spanish - the word for Tuesday is still derivative of the Roman God Mars - Mardi, Martedi and Martes - respectively). When the Germanic tribes conquered England, they laid their own lexicon over that of the Roman's so that the Norse God of War now supplanted the Roman God of War. Thus they called the day of the God of War tiwesdaeg. Tuesday
The mid-day of the week is named for the Norse God, Odin. He was also known as Woden or Wotan. Unlike many of the other days of the week, this day did not correspond roughly with the Roman designation for the day. (The Roman's named Wednesday for the messenger God - Mercury - In Romanian, the day is still known as miercuri). The early Scandinavians and Germans believed that Odin was the chief God of Asgard and as such deserved to have a day of the week named for him. The Anglo-Saxons used the word, Wodnesdaeg. Wednesday is often referred to as "hump day" because of its position as the middle day of the work week. Only one holiday typically recurs yearly upon Wednesday. Ash Wednesday is the official beginning of lent. It is called "Ash" Wednesday because since the 400's it has been the day upon which religious penitent's foreheads (fronts) are marked with ash. It is a reminder of the mortal condition of the flesh (la chair) - that we are all dust. This day is a variable date dependent on the date which Easter falls.
 Thor was the Norse God of Thunder. The Scandinavians believed his chariot rumbled as it crossed the sky and that he held a hammer that shot lightening when he threw it. The Angles and Saxon's who invaded England in the 500's carried belief in Thor with them in their wanderings and wars. In the days before the invasion the Roman's inhabited most of the civilized world (including most of England). The fifth day of the week was known as "dies jovis". The Roman's had named it for their own God of Thunder, and also chief of the Gods, Jupiter. When the Germanic tribes suplanted the resident Roman's of England they also suplanted their Gods. They replaced Jupiter or Jove with Thor. Thus came the name, Thorsdaeg which comes down to modern English speakers as Thursday. There are two holidays that traditionally fall on Thursday. One is Maundy Thursday [mɔ:ndi] (le Jeudi Saint). It is the Thursday upon the eve of Good Friday. Biblical accounts have it that the "Last Supper" was on a Thursday, and it was at this meal that Christ gave the "mandate" to his disciples to "love one another". The word Maundy is a corruption of the Latin word "mandate".
Lucky or unlucky, Friday is one of the most popular days in the week. TGIF is the commonly used acronym meaning "Thank Goodness It's Friday!". There is even a popular restaurant that goes by the name, claiming that whenever you eat there it is Friday. The love for the day is undoubtedly engendered by the fact that in the Western World it is the last workday of the week. In predominantly Moslem countries, Friday is a holy day, a day to gather in the mosques at noon for prayers and to spend the day in religious contemplation. The name of the day is derived from yet another Norse God (in this case a goddess), Frigga. She was believed to be the wife of Odin and was the goddess of marriage and the hearth. The Roman's had named this day for their goddess of beauty, Venus. They called it "dies veneris". When the Germanic tribes invaded England they imposed their goddess upon the day meant to honour Venus. The day was called frigedaeg, it has been corrupted over the centuries (since the 500's) to be "Friday". Since early times a Friday that falls on the 13th day of a month has been thought to be unlucky. Historians think that this may be because there were thirteen people present at the Last Supper of Jesus Christ and that this number proved to be unlucky; the following day (a Friday) Jesus was crucified. The Friday crucifixion spawned another Easter time holiday known as Good Friday. The appellation, "good", was likely a corruption of the word, "God". So that Good Friday was originally, God's Friday, the day Christians believe God showed his love for the world by relinquishing his son.
Saturday is the only day of the week that retained its Roman origins in the wake of the English invasions of the Angles and Saxons. This may have been because there was no Norse God to roughly correspond to the Roman God of Time and the Harvest, Saturn. The Anglo-Saxons simply adapted the Roman, "dies saturni", making it saterdaeg. Besides his ownership of a day of the week. Saturn presided over a Roman festival known as Saturnalia. It was a celebration of the harvest, but it was also a time when life was turned on its head. Crimes committed on these days often were not punished; masters frequently waited on (servir) their servants; and revelry (réjouissances) was taken to excess. Saturday is the seventh day of the week and as such was the day upon which the Hebrew God commanded his people to rest. Through to our modern age Jews keep Saturday holy as the Sabbath - quite a contrast to Roman practices. Saturdays are a popular day for weddings in Christian cultures because it is typically a day free from work and it precedes another day of rest - Sunday. (In Moslem countries - for a similar reason - Thursdays are a preferred day for weddings.) Before the advent of the 5 day work week (which began to come into vogue in the early 1900's) Saturday was much like Friday, in that it was the final workday before the day of rest. As the centuries pass conditions change the nature of these days of the week. Let’s notice the idea of Monday holidays or  Friday holidays to create 3-day weekends.

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