While it is rather clear that Sunday is the day of the sun and Monday the day of the moon, where does Tuesday or Wednesday get their names from? Are they all related to the names of the planets?

The first thing that springs to my mind is why there are seven days in the week. We are so much used to it that it may seem natural to us, but nothing related to the calendar is natural. It’s just a convention, a system humans invented to divide time, and it’s not perfect, as most things made by humans.

A T-shirt for each of the seven days of the week.
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The seven day week came into use in Roman times after the Julian calendar was introduced in 46 BC. However, both the Jews and the Babylonians had used it before: in the Bible, God is said to have created the world in six days and needed the seventh to have a rest, while the Babylonians divided the lunar month in four seven day periods.
It was the Greeks and later the Romans, who always followed in the steps of the Greeks, who started to call the days of the week after the main celestial bodies that were seen from the earth, and of course, these bodies were called after their main gods and goddesses.

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Apart from the day of the sun (dies solis) and the moon (dies lunae), they had dies martis for Mars, god of war, dies mercurii for Mercury, god of commerce, dies iovis for Jupiter, the father of the gods and responsible for thunder and lightning, dies veneri for Venus, goddess of love, and dies saturni for Saturn, god of agriculture.
Most of these names still survive in Romance languages, with some exceptions such as the change of Sunday for “the day of the lord” or dominicus dies, which gave “domingo” in Spanish or “dimanche” in  French. A different case is Portuguese, which changed the names of these pagan gods for ordinal numbers.
The Germanic peoples, however, substituted the names of Roman gods with their own, forgetting in this way that week days owed their names to the planets. They also used their own words for sun and moon. English being a Germanic language, it kept the names of these, and so we have:

• Tuesday: the day of the god Tiw or Twia, the god of war.
• Wednesday: the day of Woden or Odin. He was the carrier of the dead.
• Thursday: the day of Thor, god of thunder.
• Friday: the day of Freya, goddess of love, beauty and fertility.
• Saturday: the day of Saturn, the only Roman god they kept.

Finally, remember that it’s not the same to say “day of the week” or “weekday”. The former is any day of the week, while the latter is used for work days, that is from Monday to Friday, excluding Saturday and Sunday.

A good song to practise the days of the week is “Friday I’m in love” by The Cure. Enjoy!