In response to a fan asking if Meredith would ever be back, showrunner Myrlene King tweeted that "she's actually in Halloween. Boo. She's scary isn't she?"
If you can't remember who Meredith even is, here's a refresher: She's the student Byron Montgomery (Chad Lowe) had an affair with before the show, a secret that Aria (Lucy Hale) was burdened with for over a year until the truth came out and ripped the family apart. In conclusion, we don't like her.
Since the Halloween episode is a flashback/prequel to before Alison (Sasha Pieterse)'s death, we're guessing we'll get a glimpse of the beginning of Byron and Meredith's affair. Which: Eww. We could do without a reminder of this heartbreaking plot point.