A very interesting question!

Moreover, I must admit, sometimes (in some contexts) we do say "в этот месяц" and "в этот год" (in accusative). Try googling "в этот месяц" and you'll see the results. So, it is not only the question of singular-plural. There is a subtle nuance in that.

I do feel some stylistic difference between "в этом месяце" and "в этот месяц". The former, I think, is nearly universal. The latter is used only in some contexts. It's very hard to find any explanation for that... I've never noticed it before

Any ideas are welcome!