Translation:Fin da oggi
Both should be correct. But, can a native speaker comment on the difference in usage? Is "fin" just for emphasis? And what is it emphasizing? Or are they interchangeable?
I asked my Italian professor (a native speaker) a while after posting this. She says "fin da" and "da" are a little different. "fin da" marks a fixed point or limit. It's a bit less general than just "da". I think "da" is like "since" and "fin da" is more like "ever since".
Incidentally, "fin" and "sin" are interchangeable. You should use the one that sounds better.