Definition of Week
Work week starts on Monday. Historical (biblical) week starts on Sunday (let there be light). Sabbath (Saturday) is the last day - rest day - in the Jewish scripture, as a sabbatical is an extended rest from work.
"Next week" is thus somewhat ambiguous - as it could mean the closest Sunday or Monday.
"This week" is thus somewhat ambiguous - as it could refer to "this coming week" (next week) or "this present week" (current week). It makes more sense that "this week" refers to "this present week", and 'this coming week' is 'next week.'
However, if someone says "this week" and means "this coming week", then "Next week" could really mean "the week after next week".
In other words, do not use "this week" or "next week" if you want to make sure that you don't mess up the schedule. Instead - use the date "April 27,2017." To be more courteous, also use the day - such as "Monday, April 27, 2017."