To me, this response means absolutely nothing. Yes, I know that you use the article with the name of the day in Spanish, and I was not questioning that. The point is, this is about TRANSLATING a Spanish Statement into English, where the use of the word "on" neither makes the English statement right nor wrong, nor does it add anything at all to the MEANING of the English translation.
Furthermore, since "los lunes" translates to Mondays in general, the Mondays, all Mondays, etc. (pretty much like any article with Spanish nouns), there is no indication that the word "on" is needed at all in the translation (not needed, but also totally correct if included). In fact, "Todos los lunes" is often translated as "every Monday", with no inclusion of the word "on". And simply saying "Los lunes" even means the same thing (in both languages), without requiring the added stress of saying "todos" or "every".
So my whole point was that I see no particular reason why DuoLingo should require the word "on" in the translation, and I reported it for that reason. Sorry for being so wordy.
I take it you mean "me". It's not used instead of "yo". It is always needed whether or not you include "yo" because it forms part of the reflexive verb "levantarse" - to get (oneself) up. So you can say "yo me levanto a las seis" or "me levanto a las seis" either one is correct. It is not affected by the "on Mondays" either.