。
I wish every day were Saturday.
Literal
Every-day [subject-が] Saturday were-[conditional-だったら] good [regret-のに].
~たらいいのに expresses a wish contrary to fact — 'if only it were so.' The final のに adds longing or regret, implying 'but it isn't.' 毎日が with が (rather than は) highlights 'every day' as the focused subject of the wish.