No, nzuri is for nice/good/beautiful; mwema or njema is referring more to the character, good (as in the Good Samaritan). This is ritualized, so if the explanation does not help you, just memorize it. :) I have not heard "siku nzuri"
Siku would become "Siku njema." I haven't heard it a lot (in contrast to "Usiku mwema." which is a common greeting.)
So it simply is not in use - please correct me; I haven't heard it neither in Tanzania nor Kenya nor in class. :)