Days of the Week
The seven-day week has two unrelated sources. On the one hand, there’s the Hebrew Book of Genesis, according to which God created the world in six days and rested on the seventh day. On the other hand, there’s the Babylonian and Greek tradition of associating gods with planets, of which seven were known in ancient times. There was also an association between planets and days, hence the seven-day week, which became customary, first in the Roman Empire, and later world-wide.
What the ancients called planets is different from what we call planets. For the ancients, the sun and moon were planets, but the earth was not.
The names of the days of the week reflect a polytheistic past with astrological overtones. For reasons to be explained later, I'll start with the day corresponding to the outermost planet known the the ancients.
1. Saturday < Saturn
2. Sunday < Sun
3. Monday < Moon
4. Tuesday < Tiu (Norse god of war)
5. Wednesday < Woden or Wotan
6. Thursday < Thor (Norse god of thunder)
7. Friday < Frigga (Norse goddess of love)
Most of us are less familiar with Norse mythology than with Greek and Roman mythology. The names of certain days in Romance languages can tell us which Roman god was identified with a particular Norse god. For example, in French some of the days are
4. mardi < Mars
5. mercredi < Mercury
6. jeudi < Jove (Jupiter)
7. vendredi < Venus
Now we have a complete list of planets corresponding to days:
1. Saturday → Saturn
2. Sunday → Sun
3. Monday → Moon
4. Tuesday → Mars
5. Wednesday → Mercury
6. Thursday → Jupiter
7. Friday → Venus
The order of the planets should seem strange at first glance. For example, why should the sun follow Saturn? To explain the peculiar order of planets, I will eventually cite the marvelous book by George Sarton:
A History of Science: Hellenistic Science and Culture in the Last Three Centuries B. C. (1959)
That book is a sequel to an earlier book:
A History of Science: Ancient science through the Golden Age of Greece (1952)
First let me introduce the author. George Sarton (1884-1956) invented History of Science as an academic discipline. He was equally at home in each of C. P. Snow's two cultures. Unlike most so-called historians of science nowadays, he believed it was necessary to know something about science before writing its history. Sarton's explanation (on page 322) is as follows:
"The astrologers ... divided the [day] into 24 equal hours. Each hour was dedicated to one of the seven planetary gods, and each day was called after the god of its first hour. Let us begin with the day of Saturn (Saturni dies), so called because its first hour was dedicated to Saturn; the second hour was the hour of Jupiter; the third, of Mars; the fourth, of the Sun; the fifth, of Venus; the sixth, of Mercury; the seventh, of the Moon. Not only the first hour, but also the 8th, the 15th, and the 22nd were dedicated to Saturn. The 23rd and 24th hours were dedicated to Jupiter and Mars, and therefore the first hour of the following day belonged to the Sun, and that day was called Solis dies. Therefore the astrological order of the planets:
Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon,
was replaced by a new order obtained by jumping two items after each item of the first series. One thus obtains
Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus,
which is the order of our days.”
You can find out more about the author from his Wikipedia page:
https://en.wikipedia.org/wiki/George_Sarton
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It's a little bit complicated, but you have made it easy to comprehend.
Wiktionary gives the following etymology of "month":
From Middle English month, moneth, from Old English mōnaþ (“month”), from Proto-Germanic *mēnōþs (“month”), from Proto-Indo-European *mḗh₁n̥s (“moon, month”), probably from Proto-Indo-European *meh₁- (“to measure”), referring to the moon's phases as the measure of time, equivalent to moon + -th. Cognate with Scots moneth (“month”); North Frisian muunt (“month”); Saterland Frisian Mound (“month”), Dutch maand (“month”); German Low German Maand, Monat (“month”); German Monat (“month”); Danish and Norwegian Bokmål måned (“month”); Norwegian Nynorsk and Swedish månad (“month”); Icelandic mánuði (“month”); Latin mēnsis (“month”); Ancient Greek μήν (mḗn); Armenian ամիս (amis); Old Irish mí; Old Church Slavonic мѣсѧць (měsęcĭ
. See also moon.
And of course the plural of the Latin mēnsis (“month”) is mēnsēs (“months”), from which the English words "menses", "menstrual", etc. are derived (you alluded to this).