Dies, Diei /ˈdi.ɛːs/ (m) noun; 5th declension
It may mean the same thing as Day, but it is not a cognate. They do have similar origins, but would form completely different words were they to have been cognates. The fact that this noun is masculine is the reason why the word for day in many Romance Languages is masculine. Little pointer if you’ve wanted to take a crack at other non Latinate Italic languages.