Sun, Moon, Mars (Ares), Mercury (Hermes), Jupiter (Zeus), Venus (Aphrodite), and Saturn were named after the planets of Hellenistic astrology, in the following order: Sun, Moon, Mars (Ares), Mercury (Hermes), Jupiter (Zeus), Venus (Aphrodite), and Saturn (Cronos). The names were probably chosen by priests who were aware of their significance but not necessarily members of the family.
The first day of the week was originally called "Sunday" because the Romans believed that Zeus, the king of the gods, came down from his throne on Sunday to watch over his people. They also believed that Hades, the king of the underworld, came up from below on Sunday to take charge of the souls of the dead. As a result, all pagan religions observed their main holy days on Sunday.
Later on, when Christianity became the state religion of Rome, many changes were made to make it more acceptable to the existing system. For example, Friday was renamed "Venerable", meaning "worthy of reverence", and became the first day of the weekend. Saturday, which previously had no name, now became "Holy". Sunday remained "Sunday" though, and so did Monday. Tuesday was called "St. John's Day", because it was on this day that Jesus, according to Christian belief, was baptized by St. John the Baptist. Wednesday was left unchanged because it has always been known as "Lunar Day".
The days were named after heavenly bodies, which in turn were named after gods and goddesses in certain circumstances. For example, Monday is named after Moira, who is one of the names of Athena (the Greek goddess of wisdom).
Saturday is named after Saturn, who is a god associated with agriculture and craftsmanship. He is also called Rhea's day because he was the first planet discovered by astronomers. Sunday has no obvious connection to any deity but it does correspond to the sun coming out once more after the long winter night. This makes sense because it is the day after Saturday and things are starting to grow again.
Monday is the first day of the week because Sundays should be reserved for rest and reflection, and so the working week starts back on Tuesday.
The Romans called the days of the week after the Latin terms for the Sun, Moon, and the five known planets. Monday is luna septembria, "the seventh night"; Tuesday is luna octobris, "the eighth day"; and so on.
The language of the Romans was not familiar to most English speakers until the late Middle Ages, so it's possible that they named the days themselves. However, since they also used lunar months instead of solar, this would have made the week follow a lunisolar calendar rather than a solar one.
It is more likely that they were given names by priests who didn't know any other way to refer to these days. They may have gotten them from the Bible or some other religious text; or perhaps they just liked the sound of these words.
Today, people usually name the days of the week after God, Jesus, or saints. Sunday is sabbath, which comes from the Hebrew word for "rest"; Monday is luna mensis, "the month's moon"; Tuesday is luna octavus, "the eighth day"; and so on.
This system of naming days after events or people occurred quite early in Europe.
The days are named after planets in various languages, including French and Spanish. For example, in Japan, the days of the week are named after the five classical elements (which are associated with the planets). Many languages call Sunday after the Latin dominus, or Lord, because it is the day when people attend to church. Monday is called luna in Latin, but lunes in Spanish. Tuesday is called martius in Latin, miercoles in Spanish, and marca in Italian.
Some names of the days come from religions other than Christianity. For example, Saturday is called sabbat in English and sabado in Spanish because it was once considered a day of rest under Jewish law. Sunday is called dimanche in French and domingo in Spanish because it was once considered a holiday.
Other names of the days come from gods or goddesses. For example, Mars is the god of war and is associated with red color, so his icon often appears on stickers for Monday. Jupiter, the king of planets, is associated with blue color so he's usually seen on stickers for Thursday.
There are also words that describe the days themselves. They can be divided into two groups: those that refer to physical qualities and those that don't. Words such as "lazy" and "busy" describe behaviors we can have on certain days, not characteristics of the days themselves.
Saturday, Sunday, and Monday are named after the celestial bodies Saturn, Sun, and Moon, respectively, while the other days are called after Germanic gods: Tiw's day, Woden's day, Thor's day, and Freya's day.
The week begins on a Saturday because that was when Rome became aware that one of their emperors had died. Before then, they used to begin the week on a Friday but now they have changed it to start it on a Saturday so that everyone has two days off per week.
The Monday morning alarm went off at a quarter past six, and instead of lying there trying to go back to sleep, I got up and started my day. After taking a shower and getting dressed, I made a cup of tea and sat down to write for a little while. Then I drove into town and bought a large coffee at a shop across from where I lived. Back home, I drank it outside in my garden as I read the newspaper. Later, I walked around the block and had another cup of coffee before driving back home again. By this time it was about eleven o'clock and I thought I would eat something before going back to work. So I had another cup of coffee and some cheese and fruit sandwiches for lunch. Around three-thirty I stopped by a grocery store and bought some milk and cookies for myself and my wife.
The days of the week in Spanish come from these planets and gods. The first day of the week is named in reference to the moon, in Spanish "luna" (Lunes), the second to Mars (Marte), the third to Mercury (Miercoles), the fourth to Jupiter (Jueves), and the third to Venus (Viernes).
Monday: Luna (the Moon)
Tuesday: Marte (Mars)
Wednesday: Miercoles (Mercury)
Thursday: Jueves (Jupiter)
Friday: Viernes (Venus)
Saturday: San Juan (Saint John)
Sunday: Santa Maria (Saint Mary)
These are the only days of the week named after planets or gods. In English, Monday is called "Moon Day", etc.
The order of naming of the weeks was fixed by law in Spain when Sunday was assigned as the first day of the week under Roman Catholicism. Before then, the Spanish months were named after saints' days, and so the names of the weeks could have changed accordingly.
For example, January was originally called "Qarthegen's Month", after its founder, Qarthius. February was called "Pridie's Month", after its founder Priidius. And so on.