Which day does “next Tuesday” refer to?
Suppose today is Tuesday. What do last Monday, this Monday and next Monday mean?
Let's analyze this Monday first.
Obviously, the speaker means yesterday.
If it were Friday today, you'd be sure the speaker refers to the upcoming Monday. If it's Tuesday, you would think that too, but you might want to confirm the date to make sure the speaker hasn't mixed anything up. Why is that? I think that's because the next Monday is somewhat too distant from this Tuesday to be called this Monday.
Not let's look at next Monday.
Of course the speaker means the upcoming Monday. Now imagine it's Saturday or Sunday today. The same sentence should seem a bit strange. Of course, logically next Monday is the same upcoming Monday, but again, you might want to ask to specify what the person means just to make sure.
Similar considerations can be applied to last Monday.
Now, do I have any source to back up my answer? No, I do not. But does that matter? I'm pretty sure a lot of people would understand different things when hearing the ambiguous this, last, or next Monday. So, even if there were a strict rule (which I doubt there is) what good would it be if the terms are still ambiguous? Language is not mathematics, it's very ambiguous.
English has only tenuous rules for this kind of expression; i.e, it varies a lot from day to day and from speaker to speaker.