Christmas in #12 Grimmauld Place was the last thing you expected. You thought you'd be at the Burrow, Home, or at Hogwarts. But no, Harry brought you to Grimmauld Place to spend Christmas with Sirius Black: The wanted man who escaped Azkaban, who was also Harry's Godfather...
At first, you were.. absolutely terrified of Sirius. But, after an hour or so of friendly welcome and funny jokes, you warmed up to Sirius pretty fast. You realized he isn't an escaped convict. He's just a man who had 12 years of his life robbed from him.
On Christmas morning, you woke with a start from a screeching of a woman who was acting as if she was being tortured. She was screeching of "Filthy Mudbloods, traitors to my flesh!!" or, something along those lines.
Stupid portrait of Mrs. Black, You thought silently.
When you headed downstairs, you soon realized that Ron triggered the old hag's portrait, as he was running around tirelessly, shouting that it was Christmas. You saw Sirius laughing uncontrollably, seeming as joyful as Christmas itself. Seeing Sirius so.. happy.. It warmed your heart. You've never seen him this happy before. It was Sirius who had decorated the entirety of #12 Grimmauld Place, too.
When it was time to open gifts, you noticed everyone was there. Tonks, Remus Lupin, Moody, Harry (Of course), Hermione, and the entire Weasley family- except Percy, as he didn't believe that the Dark Lord was back.
Everyone had an aura of excitement, and happiness- even Moody! But Sirius, by far was the most excited. He had the biggest grin on his face. But, everyone had a wary eye on you, since you're a Slytherin. They don't quite trust you... Everyone in the room was Gryffindor (Or Hufflepuff in Tonks's case).