"Which day is it?"
Translation:Vilken dag är det?
Because det is not referring to the day here, it’s just a formal subject that is added to the sentence cause it needs a subject. You can compare it to ”is it snowing?” where the ”it” doesn’t really reflect anything.
It sounded better with the det, but for some reason I got the impression from the notes that if the noun referred to was mentioned before it should match the gender. I'll have to go back and review my notes to see where my error was. Thanks for answering.