Where do the names for days of the week come from?

The Roman system of seven-day weeks, each day named after the planets as they were then understood (hence Sun, Moon, Mars, Mercury, Jupiter, Venus and Saturn), was introduced to Britain in the early 1st century AD. It was maintained well into the post-Roman period, when certain key names were replace with those of Germanic deities, hence Tiw's day, Woden's day, Thor's day and Fríge's day.

Unfortunately, as the prehistoric Britons did not record anything in writing prior to the Roman arrival, we have no idea how they named specific days nor how they ordered their calendar.