The seventh day translates as 'sabbath', I believe; the Sabbath is the seventh day (duh!). Therefore, I agree with Nigel that the 'day of rest' is meant to be the seventh...

In the Bible there is a pronouncement by (I think) Paul who mentions that people are arguing about which is the day of rest; he says that the sabbath was made for man, not man for the sabbath and therefore it is not important.

That is not at all the impression I get from what they said earlier was God's take on it: Remember the Sabbath Day, to keep it holy'.