"Today is calm."
Translation:Mālie kēia lā.
because in this sentence, "kēia lā" is the subject, like: "This day is calm." Compare with a sentence like, "Pehea ke anilā i kēia lā?" or "How is the weather on this day?" where today ("this day") is not the subject, but extra information at the end about the sentence's location in time.
For me, it helps to think of the phrases as the more literal translations of "this day" and "on this day," instead of just "today" because in English we use the word "today" for both functions without any prepositions or markers. :)