The days in a week are named after the sun and the moon and the only five planets visible before the invention of the telescope: Mercury, Venus, Mars, Jupiter and Saturn; which were identified with Roman gods (including one goddess: Venus). This gives us seven, which, being a prime number, has magical significance. In English, Mars, Mercury, Jupiter and Venus are replaced by the Germanic gods: Tiw, Woden, Thor and Freya.
If we look at the picture below, starting at the top and moving anticlockwise, the sun-god and the moon-goddess are easy to identify, giving us Sun-day and Moon-day. The third figure in the war-helmet is obviously Mars, giving us Mars-day, or "Mardi" in French. In German mythology the wargod was called Tiw or Tiwaz or Tyr, giving us Tiw's-day.
The next picture anticlockwise is Mercury, messenger of the gods with the rather cute little wings. He gives us "Mercredi" in French. Tacitus tells us that the German equivalent of Mercury was Woden, so this gives us Woden's-day
The next figure, continuing anticlockwise, is Jupiter, the king of the Roman gods, wielder of thunder and lightning. The thunder-god in the German pantheon was Thor; so we have "Jeudi" in French and Thor's-day in English. But the last part of the mosaic is a little puzzling.
The cowled figure who completes the circle would appear to be Saturn, the god of old age; hence Saturn's-day; which would mean that the goddess in the centre must be Venus ("Vendredi" in French; the Germanic equivalent being Freya, goddess of fertility; hence Freya's-day). But why is she in the centre, out of sequence? Was the Roman who had this mosaic made perhaps a particular devotee of Venus?
P.S. I have since learned from Mary Beard's book on Pompeii that traders in the town referred to markets as being held on "Saturn's day", "Sun's day", "Moon's day", Mercury's day" and so forth. The Romans did not officially have a seven-day week, but perhaps found the idea more convenient to use than their official very cumbersome system of dating.