The Spanish word 'lunes' (meaning 'Monday') comes from the Latin phrase 'dies lunae', which literally meant 'day of the moon'. This Latin phrase was formed from two words: 'dies' meaning 'day' and 'luna' meaning 'moon'. Over time, the Latin 'dies lunae' evolved into the modern Spanish 'lunes', dropping the 'dies' portion and maintaining the lunar reference.

A related and simpler Spanish word is 'luna', which means 'moon' - exactly the same as its Latin ancestor. This makes it easy to remember that 'lunes' (Monday) is connected to the moon, just as it was in ancient times. Another related word is 'lunar', meaning 'relating to the moon'.

English speakers can connect 'lunes' to several familiar words that come from the same Latin root 'luna'. These include 'lunar' (relating to the moon), 'lunatic' (originally meaning someone affected by the moon's phases), and 'lunation' (the time between one new moon and the next). The connection between Monday and the moon also exists in English - 'Monday' comes from Old English 'monandæg', meaning 'day of the moon'.