Since the week has appeared thanks to the moon, it is not surprising that this heavenly object appeared in the name of one of the days, namely Monday.
In the European tradition, the week begins precisely on Monday, but everything is not so unambiguous like in performing assignments here. For example, in America, both in the north and south, you can often find calendars starting on Sunday. This is probably related to the Bible. As we remember, God created the world in 6 days (from Sunday to Friday), and on the 7th day (which fell on Saturday) took a rest. The same tradition exists in a number of Muslim countries (for example, in Egypt, Iran, and dozens of others) – moreover, the whole country's population really goes to work on Sunday and works until Thursday.
Tuesday received its name from the little known to us but apparently influential in former years god Tiw or Tyr. Tiw was no less than Odin's son, he lost one hand and was the god of military valor.
What is the remarkable feature of Tuesday? For example, this is the day of elections in the United States. As it turned out, the idea to hold elections on Tuesday has religious roots (like almost everything associated with these days). The vote on Tuesday allowed the Americans of the 18-19th centuries, firstly, not to miss Sunday's church attendance, and secondly, left the whole Monday to be the way to the place of voting.
There is a "black … (name of the day of the week)" for each day of the week, dedicated to this tragic event or another, but it is Black Tuesday here that stands out especially. On Tuesday, 29 of October in 1929, the famous stock market crash on Wall Street, which caused the Great Depression, changed the history of the States forever.
Wednesday – If in the case of Slavic languages, the word "middle" played a key role (meaning the middle of the working week, obviously), then the English name for this day came from the name of the god Odin. This name sounded like Woden in the ancient Germanic language, and this strange letter D in this word is from there.
Wednesday has an informal name – hump day – which reflects its middle nature, the hump is a hill, which we successfully pass by the end of the day.
Thursday is a fishing day in the Soviet tradition. This day got its name in honor of Odin's son, Tor with the hammer, which is easily guessed by the name. But there are no interesting expressions related to Thursday.
Friday received its name from the goddess Frig. The goddess is so little known to us that, in despite the fact that Friday was named in her honor, almost nothing is known about her.
Friday is one of the most mysterious days of the week. To begin with, the sailors considered it a bad sign to sail on Friday in the 19th century. There is even a city legend about how the British government, trying to refute stupid superstition, sent on the 13 of Friday the ship "Friday", which was headed by Captain James Friday. It is clear that the ship's tracks were never seen again, and no one was able to find it – perhaps because it is a city legend that has no basis.
Friday – that is how Robinson called the first person on the island. From here the expression "Man Friday" came in English, denoting a close mate, the right hand.
Black Friday – despite the gloomy name, it is not the day of the tragedy, but the biggest day of sales in the US, the day after Thanksgiving. Buyers storm shops this day, hoping to grab goods with a huge discount.
Casual Friday – another reason to love Friday, you can come to work in the office in a shirt and torn jeans (or in a not very strict suit).
Saturday, the only day whose name came from the Roman tradition, and not from Germanic, as the rest of the days – Saturn's day.
The most interesting thing on Saturday is that the next day you do not have to get up early, so bars, clubs and restaurants work late. Popular entertainment shows are on the TV. Of course, this applies only to those countries for which Sunday is a day off. But despite the fact that Saturday is perhaps the best day of the week, it left not so many tracks in the language.
You can read here about other versions of the topic under consideration.
That is all, it is time to finish the story. Good luck with learning English, whatever day it happens.