Days of the week were named from the name of the planets.
It's easy to understand this if you know the translation of English days into french.

Let me help (french is my cup of tea Tongue )

First in English
  1. Sunday = Sun
  2. Monday = Moon
  3. Tuesday = Mars
  4. Wednesday = Mercury
  5. Thursday = Jupiter
  6. Friday = Venus
  7. Saturday = Saturn
Now in French
  1. Dimanche = Soleil (sun)
  2. Lundi = Lune (moon)
  3. mardi = mars
  4. Mercredi = Mercure
  5. Jeudi = Jupiter
  6. Vendredi = Venus
  7. Samedi =Saturne
As you can see days of the weeks in french have almost the same spelling as the planets.

Now why was the days of week based on the planets?
Well this is because, each hour of a day is influenced by one planet, so for 7 hours you have 7 planets that go by the sky. If the 1st hour of the day is influenced by Saturn, the second hour will be influenced by Jupiter etc.

Here's a list of how it goes:
  • 1st hour : Saturn
  • 2rd hour = Jupiter
  • 3rd hour =Mars
  • 4th hour = Sun
  • 5th hour = Venus
  • 6th hour = Mercury
  • 7th hour =Moon.
Then the 8th hour will be Saturn again and so on repeat the list.
now the 25th hour will be the 1th hour of the next day, and you will notice that the Sun will be on that hour.
I mean this:
  • The 25th hour = sun ( or Sunday)
  • The 49th hour = moon ( or Monday)
  • The 73th hour = Mars ( or Tuesday)
and so on.

So now we understand why days were named so! (for those who didn't know)
It's in short because of the first planet that rules the 1th hour of that day.

This helps to understand why each hour and each day, is never the same, people's mood will change, as at 12h:00 you are enjoying something (Venus), and at 13h:00 something bad occurs and you are angry( Mars).
The same goes with days and months and years and centuries, things will never be the same, as long as planets are in motion.