"I was just talking about Harry's exceptional potion-making! Some credit must go to you, of course, you taught him for five years!"
--Horace Slughorn, speaking to Severus Snape (DH15)
To the dismay of Romilda Vane and other admirers of the Chosen One, Harry attends with Luna Lovegood; Hermione brings Cormac McLaggen to annoy Ron. Also in attendance are a vampire, sundry notables “collected” by Slughorn, Professor Snape, Professor Trelawney, and a wait staff of House Elves. Hermione leaves Cormac under the mistletoe, and keeps avoiding him. Harry excuses himself from a conversation with a man who would like to write his biography, but soon ends up talking to a drunk and bitter Trelawney. Slughorn praises Harry’s potion skills and Snape is suspicious of this. The awkward party interactions go on for a while, then Filch, with Draco in tow, disrupts the party and explains that Draco, whom he caught hanging around a corridor on the next floor, claims to have been invited to the party. He then admits he wasn’t invited. Slughorn favors leniency and says Draco may stay (It’s Christmas, after all.), but Snape is angry and takes Draco out of Slughorn’s abnormally large and decorated office. Harry puts on his Invisibility Cloak and follows them.
This takes place on the evening of the last day of classes before Christmas break (HBP15).
Draco was hanging around the fourth floor because he needed to get into the Room of Requirement to work on the vanishing cabinet. He looks so unhappy about being allowed to stay at the party because his statement that he had been trying to crash was a lie to cover for his real activities, and now he has to stay so not to blow his cover (HBP27). Snape is upset not because Draco was roaming the halls or barging into parties, but because he knows the stakes of Draco's actual situation (HBP15).
Trelawney appears to be genuinely interested in Luna's wild ideas and take them seriously. This isn't entirely surprising, but it is interesting, nonetheless. -BB