In Harry Potter and the Prisoner of Azkaban the kids get their exam results at the end of the year.
The exam results came out on the last day of term.
(Prisoner of Azkaban, Chapter 22, Owl Post Again).
This is true not just of the younger yeargroups but of the people taking O.W.L.s and N.E.W.T.s as well. Fred, George and Percy's results in these qualifications are mentioned, for instance.
Percy had got his top-grade N.E.W.T.s; Fred and George had scraped a handful of O.W.L.s each.
(Prisoner of Azkaban, Chapter 22, Owl Post Again).
However, in Half-Blood Prince Harry, Ron and Hermione get their O.W.L. results delivered to them by owl (fittingly enough) in the middle of the summer holidays.
What explains the difference in timescale here? Can we tell whether Percy, Fred and George got their results early or Harry, Ron and Hermione got theirs late?