Not quite. Is it an anniversary?

Genau...an diesem Tag sagte sie zum ersten Mal "I love you" =)

Einfach nur That day!

You're close ... I'd stick a "for" in there:
On this day, you said for the first time, "I love you."

Here are some ideas (you may want to be more specific):
-- It was on this day, a year ago, that you said to me, "I love you."
-- Today, X years ago, was the very first time you said, "I love you."

Dach!

Vielen Dank ;)