[Vicomte13]

"I always thought that God rested on the seventh day."

Yes.
And the Seventh Day is Saturday.
Which is why the Sabbath, in the Bible and in Jewish practice, falls on Saturday.
The First Day is Sunday.

[AmusedBystander]

Which is why the Sabbath, in the Bible and in Jewish practice, falls on Saturday.

So why did Christians change it to Sunday?

[Reeses]

The days were named in the order of distance of celestial bodies "orbiting" Earth: Moon, Mars, Mercury, Jupiter, Venus, Saturn, Sun. Moon-day is the first day, Saturn-day is 6th, Sunday 7th. This was the original intention. Sunday is on the week-END, not the week-start.

Deci means 10 so at some point December was the 10th month, Sep means 7, Oct means 8, Nov is 9. These days it's all messed up.