I sense that this should be Mondays – in the plural – in English (at least in British English). The person speaking presumably doesn't like >all< Mondays. The example sentence might be correct, say, if you were discussing plans next week. 'Tuesday will be alright, but I don't like Monday,' but that's an unlikely context.
I agree with that. But the problem is, that we are unable to change the default Polish sentence (to say "poniedziałki", Mondays). And imperfect how it is, only "Monday" is a translation of it. So it has to stay.
We won't let such awkward sentences happen in the new version of the course ;)