"Σήμερα είναι μία διαφορετική ημέρα."

Translation: Today is a different day.

[Prettystone]

Αύριο είναι μία διαφορετική ημέρα

[D_..]

Αυγό είναι μία διαφορετική ημέρα

Αυγό = Egg
Αυτό = It/This ;)

You cannot say 'Αυτό είναι μία διαφορετική ημέρα.' It is ungrammatical in Greek. Furthermore, you cannot replace 'today' with 'it/this/αυτό'.

[Prettystone]

Eh, i always make confusion between egg and tomorrow :P
Edited.

[D_..]

Regarding your edit:

Αύριο είναι μία διαφορετική ημέρα

Αύριο = tomorrow. So, still not a good translation I'm afraid. Unless you're quoting Scarlett O'Hara, I suppose...

[Prettystone]

That :)

[JOSHOCLOCK]

Why doesnt Σήμερα need an article?

[G.Georgopoulos]

Why should it have an article? "Ημέρα" is a noun, but "σήμερα" isn't, just like "day" and "today".