The answer really depends on which faith you ascribe to. And of course, if no religion is involved, then I suppose any day could be considered a day of rest if desired.

The Jews observed the Sabbath, or their day of rest, from sundown Friday night to sundown Saturday night. It was done so in keeping with the creation of the world, where God created the Earth and all things on it, including man, in six days, and rested on the seventh.

After Christ came, taught his gospel, and was crucified, the day of rest for Christians was moved to the first of the week, or Sunday, to recognize when Christ arose from the dead or was resurrected.

So, the observance of the rest day as far as religion goes really depends on your preference. There are others, like Seventh Day Adventists, who believe the Sabbath should still be Saturday.

My understanding is that at least in some countries, the Muslim religion has their holy day or day of rest on Fridays.