Hi,
This one always gives me pause, and I find many "native speakers" are equally confused. Some people are confident in their interpretations, which always seem to differ from someone else's.
Today is Tuesday.
"Monday was the last day of summer." (reply) "This Monday?"
"No, last Monday." (reply) "Do you mean yesterday, or last week?"
If today were Friday or Saturday, we'd be more likely to assume "
Monday was" referred to Monday of this week. So would "last Monday" - I think.

But if Monday was yesterday, we're reluctant to call it "last Monday." It's very confusing.
Does anyone feel confident about this?
Thanks, - A.