You just unearthed something I never realized about my own use of "this" and "next".

I don't think I've ever uttered the phrase, "this Monday". Ever.

The soonest Monday in the future is always "next Monday", because it's always in the following week. So, if I'm talking to you on a Thursday, I'll say "next Monday" to refer to the day four days hence. If we're talking on a Sunday, then it's just "tomorrow".

But with a day like Friday, the this/next thing comes into play. I'll use "this Friday" to talk about the future Friday that shares the week we're in, and "next Friday" to refer to the one that occurs in the following week. So for example, June 12, 2021 will become "this Friday" tomorrow morning. But right now, it's still "next Friday", because the weekend ain't over.

This comment is mostly me trying to add structure to a stream of consciousness realization that just hit me. Reading the phrase "this Monday" knocked me off balance a bit and I couldn't articulate why at first.

If it's Saturday, and you're sending an email or text to confirm Monday's plans, you'd more likely to say "this Monday" than you would say "next Monday," though?

I agree with you, "this <weekday>" means the upcoming day this week, and generally changes to "last" if referring to the past, and "next" if referring to next week. "Next week Monday" or "a week from Monday" could both refer to "Monday, a week from now" though the first one might be confused if sent on a Friday...

Really, the answer is to skip English entirely and use calendar events and hope time zones don't change between when you plan the event and when it occurs. ;-)

If given a binary choice, I think you're right. I'd use "this Monday." But really I'd just say "Monday" or "on Monday" in that case. As in, "Just confirming that we're going to update the confabulator settings on Monday."

After writing my previous comment, I keep coming up with exceptions and variances to those exceptions. Like—as I write this—I would refer to June 9 as "this Wednesday", but June 11 as "next Friday". So I suppose I "glue" Fridays to the weekend? And right now it's still the weekend so June 4 retains the "this" title until tomorrow?

Starting to feel like I have epicycles of rules similar to the adjective ordering we all know, but never explicitly learned ("big red bouncy rubber ball" is okay; "bouncy red rubber big ball" is NOT OK)

Yeah, the "weekend-ness" of Friday is interesting. Actually I could extend that to the last couple days, in relative time. For example, if it's Thursday, and I said "last Monday", that would actually be kind of confusing, just as if it were Sunday and I said "last Friday". I'm much more likely to say "On Monday," or "On Friday," if it's within the current week or the current weekend, and use the sentence context to figure out if I meant the previous weekday or the upcoming one.

I'm glad I don't write either dictionaries or voice-based/natural language apps for calendar appointments. ;-)

"this coming Monday" and "following Monday" are better!

And in practice, "this coming Monday" and "not this coming Monday, but the following one"!

I personally use "incoming" and "past" instead --- indicating the closest one in the future/past.

"Monday week" clears that up.

TIL. I had never heard that until now (I'm American). Seems to be a British idiom?

Brit here: I've only ever heard that used by Americans. YMMV.

It's a common phrase in East Anglia in my experience.