When was the first sunday?

When was Sunday created? I mean, we have Sunday once a week , so every 7 days, who decided that? The same thing for Islam, who decided that the holy day is Friday, that happens to be every 7 days? I looked it up and google says "321 CE Emperor Constantine" I don't know much about religion, so, if Jesus was born in year 0 (Emperor Constantine wasn't alive yet) then, was Sunday somehow related to the lunar calendar?