Chapter 12 - "The Mirror of Erised" - In which Harry, Ron and Hermione (HRH) start searching the library for facts on Flamel, the school empties for Christmas, Ron teaches Harry wizard chess, Harry gets his father's invisibility cloak for Christmas, tries it out at night and finds the Mirror of Erised, sees his family and is cautioned by Dumbledore.
As Christmas approaches, snow covers the grounds and the castle begins turning cold. Malfoy makes fun of Harry for having to stay over Christmas, but Harry thinks about how excited he is not to have to return to Privet Drive.
An Eventful Christmas
Hagrid walks up carrying a Christmas tree, and soon Malfoy arrives and starts making fun of Ron, whose attempt at retaliation loses him points from Snape. To cheer him up, Hagrid invites the trio to see the beautifully decorated Great Hall and they tell him they've been looking for information on Nicholas Flamel.
Before falling asleep, Harry decides to use his Invisibility Cloak to walk the castle, heading for the [Restricted Section] of the library. After almost being caught and running from Snape and Filch, he accidentally discovers the Mirror of Erised, and sees his family for the first time.