the timeline of The Prank is something like this: it happened when they were sixteen so it means it has to have happened either in year five or six, anyway between 3rd of november 1975, when sirius turned 16 and 27th of march (most probably, we don't know peter's birthday) 1977, when the one who is presumed the youngest turned 17. snape's worst memory happened in,,, june 1976 (merely hours after their dada OWLs), when sirius was almost 17 so it's unlikely that it did happen after june, since holidays were coming in a short time. looking at it this way it has to have happened a little before snape confronted lily about james liking her and, in turn, lily confronted snape about his friends.
"they don't use dark magic, though." she dropped her voice. "and you're being really ungrateful. i heard what happened the other night. you went sneaking down that tunnel by the whomping willow, and james potter saved you from whatever's down there--"
