I think Fourth Year would work best, some time after the Triwizard Tournament is announced. Luna could be put in charge of the Newsletter for the school regarding news about the Triwizard Tournament, including reporting on the other school's arrivals. Originally, she was tasked on writing it herself, but say, Draco decides to be a bit cocky and attempt to put his name in the Goblet of Fire, and Severus stops him. As the Head of Slytherin house, he is not amused nor impressed by the Fourth Year's attempt to be a show off, and since he's only dragged him just out from the Great Hall to the Grand Staircase, the first person he spots is Luna who is writing her notes for the Newsletter. Snape of course takes no nonsense and to show he's impartial (And to remind his own Slytherin Students to not be so foolish to try and put their names in the Goblet), the Head of Slytherin calls Luna over and explains that Draco is to help with the newsletter. To put it bluntly "If you wanted to be so involved with the Triwizard Tournament...So be it."

Trying to think of what to do about Ginny. If I remember right, she has a relationship with someone later, and the only reason she breaks up with that someone is because Harry had drunk Felix Felicis (Liquid Luck) and along the way whilst he was still under the potion's influence, he manages to destabilise Ginny's relationship unintentionally. So we can just tweak it so that never happens and Ginny ends up happy with...That someone whose name I've still forgotten. That happens in the Sixth Year (So for Ginny, her Fifth Year), and I'm not sure about the Fifth Year. But in the Fourth Year (Ginny's Third Year), she goes to the Yule Ball with Neville and ends up with someone else I think. And if Ron's going to slowly turn into our antagonist, surely he won't suggest his sister as a potential date for Harry.