There is an instinctive, but actually rather complex, series of rules that goes into deciding when you use ‘this’ versus ‘next’; complex enough that confusion often arises when talking about days of the week occuring in the future.  

The general rule:

1. Insert week’s after any instance of this or next.

For example: 'This week's Monday' instead of 'This Monday'

More examples:
I’ll have it done by next Wednesday.
            Becomes…
I’ll have it done by next week’s Wednesday

I’ll have it done by this Wednesday
            Becomes…
I’ll have it done by this week’s Wednesday

The advantage of this rule is it eliminates confusion. The disadvantage is that it is not always necessary and sounds odd when it isn't.

The Weekly Cycle
Humans base their concept of dates on the weekly cycle. A person 'places' themself in the current week, so this week's wednesday refers to the same wednesday regardless of whether it occurs in the past of the future.

Complication arises around Sunday because, mentally, the cycle begins on Monday and ends on Sunday, but officially, it begins on Sunday and ends on Saturday. See the footnote for more details.

Dropping the This / Next:

Origin on Monday
Understanding this/next is easy if today is Monday, because the whole week is nicely laid out before us and every day can be called ‘this’ (or with no prefix at all), but things start to fall apart as soon as we slip deeper into the week.
If today is Wednesday and I want to talk about Monday, there is an inherent confusion about whether I am talking about the Monday that just passed (this week’s Monday) or if I am talking about the Monday yet to come (next week’s Monday); therefore I may never simply call it ‘this Monday’ if it occured in the past.

The Past
Generally the context of the conversation already includes whether we are talking about the past or the present but if it isn’t and if you actually want to talk about the Monday that occurred in the past, you always need to refer to it as last Monday.  If it occurred farther in the past, you need to say: the Monday before last Monday.  Any further back than that and you’d better be prepared to get a calendar and call it Monday January 12th, or whatever it was.
Be prepared that whenever you say “Last Monday” the person you are talking to will inevitably respond with “Three days ago?” or something similar, to which you will confirm “Yes” and the conversation will move on.

Other rules

1.  always refer to tomorrow as ‘tomorrow’ and yesterday as ‘yesterday’
2.  if it is next week, always use ‘next’
3.  if it is next week, and over 7 days away, use ‘after next’. (the Tuesday after next)

An example week (where today is Wednesday)

notice that, in this example, it is not acceptable to say 'Monday', 'Tuesday', 'Wednesday', or 'Thursday' without prefix.

WTF, the week starts with Sunday, not Monday!
Our calendar is cobbled together from different conflicting sources.  This means it is far from perfect. Officially, the week starts on Sunday, but time is a continuous process, weeks string together endlessly and arbitrary delineation points are not going to ‘stick’ in the collective consciousness the same way that the rest-work division of Sunday Monday does.  In adult conversation only assholes point out that the week starts with Sunday, so please, don't embarrass yourself.