The exciting story as to how the days of the week got their name stretches back to Ancient Greece and is covered in layers of history, religion, astronomy and even maths.
The Ancient Greeks named each of the days after dieties and planets. The Romans did the same thing when the Julian calendar was adopted in 45BC
Monday: Moon – Ἡμέρα Σελήνης (Greek), Dies Lunae (Roman), Selena (Germanic)
Tuesday: Ares – Ἡμέρα Ἄρεως (Greek), Dies Martis (Roman), Tyr (Germanic)
Wednesday: Hermes – Ἡμέρα Ἑρμοῦ (Greek), Dies Mercuri (Roman), Odin (Germanic)
Thursday: Zeus – Ἡμέρα Διός (Greek), Dies Iovis (Roman), Thor (Germanic)
Friday: Aphrodite – Ἡμέρα Ἀφροδίτης (Greek), Dies Veneris (Roman), Frige (Germanic)
Saturday: Kronos – Ἡμέρα Κρόνου (Greek), Dies Saturni (Roman), Saturn (Germanic)
Sunday: Sun – Ἡμέρα Ἡλίου (Greek), Dies Solis (Roman), Sun (Germanic)
The European Romance Languages (France, Spanish and Italian) have kept the gods hidden in the days of the week, however as Christianity spread, the clergy no longer wanted the days of the week to be named after pagan gods.
In the Greek-speaking East they used numbers, counting on Sunday as ‘the Lord’s Day’ or Κυριακή as the first day of the week, followed by Monday (Δευτέρα – the Second), Tuesday (Τρίτη – the Third), Wednesday (Τετάρτη – the Fourth), Thursday (Πέμπτη – the Fifth), Friday (Παρασκευή – The day of preparation) and Saturday(Σάββατο – the Sabbath).