Let's analyze this Monday first.

I saw him this Monday.

Obviously, the speaker means yesterday.

I have a meeting with him scheduled for this Monday.

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.

I will see him 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.

Let's analyze this Monday first.

I saw him this Monday.

Obviously, the speaker means yesterday.

I have a meeting with him scheduled for this Monday.

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.

Now let's look at next Monday.

I will see him 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 that 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.