Are there many people who pay for 3 Saturdays and then skip one? I would be surprised.
What age is the target group? An adult person can probably easily find 3 free Saturdays in a row. For a student living with parents it will probably be more difficult, because it means 3 weekends when the parents cannot organize any whole-weekend activity.
What would be your proposed alternative to Harry Potter fanfiction? Something else fan fiction? Original fiction? Not a fiction?
As I see it, choosing the Harry Potter universe that many readers already know allows it to use the contrast between how the different versions of Harry Potter behave.
Harry Potter in the books is frankly an idiot. Every year the most powerful dark wizard is trying to murder him, and he always only survives because he gets lucky… and yet the only thing he worries about is quidditch. Voldemort doesn’t seem very smart either; his only smart move in the entire series was making the horcruces. Otherwise he is just a bully who can say “avada kedavra” faster than anyone. It seems like if he really tried to kill Harry, then Harry wouldn’t have a chance. Seriously, if you can possess people, including teachers at Hogwarts, how difficult it would be to say “Harry, come here” and then stab him with a knife? The entire society of wizards seems to have a huge collective blind spot about doing anything that does not involve magic. Which is weird, considering that many students are coming from muggle families.
So I guess if your message is that people are idiots, it can be well delivered in a fan fiction from the universe where all people are idiots, and the book had millions of readers who didn’t mind this fact, probably because in similar circumstances they also would have acted the same way.