What is the next of Monday?

“Next Monday,” in the strictest interpretation, means whatever Monday will happen closest in the future, excluding the one you currently occupy if it happens to be Monday.

Do I say this Monday or next Monday?

I have always used “this Monday” to mean the coming Monday, or the next Monday to occur on the calendar. So, if it is Tuesday and I say “I will be ready for this Monday”, I mean I will be ready on Monday the following week. “next Monday”, then, refers to the second Monday occurring on the calendar in my usage.