The English translation uses "On Mondays" to imply that I never work on Monday. Is this really what the Russian sentence also means? How would that change if I mean I do not work (this) Monday. Note that I can say (maybe somewhat colloquially) in English "I do not work Monday" to mean this coming Monday without actually specifying "this"
Hi! Usually when you say "В", it means that you don't do an activity regularly.
Я иду в магазин В понедельник – I'll go to the store on Monday (Идти (иду) is an imperfect verb, used for uncommon situations)
If you say "По", it typically means that this activity is regular for you:
Я хожу на работу по понедельникам – I go to work on Mondays (Ходить (хожу) - perfect verb tense, used for situations that happen routinely)
Hope it helps you! :)