"Truly" is the opposite of "wrongly" - it does not express degree. Here the statement is "Today is not my day" and "proprio" needs to be translated to emphasise how bad the day is. The best English translation is "just", which has to come before the word it stresses, i.e. "not". Hence: "Today is just not my day."
I put "Today is not just my day." which seems like an appropriate answer for the given text, even though it has a different meaning than "Today is just not my day." I got it wrong due to the double negative DL answer which is absolutely wrong and I've reported it. I am curious about my answer. I'm wondering if "proprio" would go at the end of "il mio giorno" since the emphasis in my answer is on "my day" (or "my") rather than "just".
Today is not just my day or today is just not my day is the same so how can this be wrong? This sentence in English is not a sentence one would speak or write. This section seems to create many difficulties and ambiguities. Could it be there are errors for Duolingo with these translations? I have never has theze problems with other sections.