Why is it "nie piję kawy" instead of "nie piję kawę"? Do you not have to convert the noun into Accusative here?
Direct objects in negative sentences always take take genitive case:
The verb "pić" (to drink) takes a direct object in Accusative.
However, if you negate a verb that normally takes Accusative, it takes Genitive instead. (piję kawę -> nie piję kawy).
This is the only case which changes when negated, other cases stay unchanged.
Why you can say "w sobotę" (that is in acussitve) but in the months you have to use the locative?
Possibly the same reason why weekdays use the preposition "on" and months the preposition "in" in English. No one knows :)