Why is Sunday observed as holy day, instead of Saturday?

Why is Sunday observed as holy day, instead of Saturday, as it was in times of Jesus and before? I've looked up Wikipedia, and remember some story about a Roman Emperor (3rd century?) installing that. But, looking it up again, I cannot find this any more. Obviously, this has to do on how to count the days of the week, resting on the seventh day. --- It was proposed that https://christianity.stackexchange.com/q/52287/64221 would answer this question, but, no. Though it sheds some light, the question there is focused on "when", mine is on "why".