What's really interesting is that Portugese is one of the few European languages which doesn't follow this pattern, since at some point someone decided it was heretical to name days after those silly Pagan gods.
Domingo is the Day of the Lord, Sabado is based on sabbath and the others just get numbers. That's the trouble with monotheism, there's too damn many days and not enough gods.
That version is being used since the time of the Roman Empire, before that, Greeks used the babylonian model that named days after planets -which in turn had been named after gods.
The english names are of germanic/norse descent, again mostly names of gods.
So they have:
Domingo is the Day of the Lord, Sabado is based on sabbath and the others just get numbers. That's the trouble with monotheism, there's too damn many days and not enough gods.