Hermione hadn't wanted to go to Slughorn's Christmas party with McLaggen, much less date him, but she needed to get rid of Ron and Lavender. They were snogging everywhere she went, and they were driving her nuts. Besides, Ginny had suggested this tactic, and she could hardly say no. In a way, it made sense.
Hermione knew they were allowed to bring guests. She did have someone in mind....But no. She couldn't let him distract her from her plan.
Draco couldn't stop thinking about Slughorn's party. He had not been invited. Hermione Granger had. It was as simple as that. But for some reason, the fact stayed in him like a virus, impossible to get rid of. This was not something Madam Pomfrey could cure with a swish of her wand or a bottle of medicine.

On the night of the party, Draco couldn't stand it anymore. He would have one peek in, just to see if what Blaise had said was true, and then he would be out. No harm done. And perhaps he could stay if Slughorn was feeling generous that night.
Draco crept through the halls, wishing that he had an invisibility cloak like Potter. That would make things so much easier. He couldn't even see. He would've lit his wand, but that would attract Filch.
"What do you think you are doing out of bed, boy?"
Draco jumped. His mind was blank, and he could not think of any lie to say. This wasn't like him. He usually had no problem with this.
"I-I'm late to Slughorn's party!" he squeaked.
"Oh, are you? We'll see about that," Filch snickered. "Three weeks' detention, boy, if Slughorn doesn't agree with you, piece of filth."
Filch grabbed the collar of Draco's robes an tugged him in the direction of Slughorn's office. When they reached their destination, Draco could hear voices and music playing, laughter and drinks clinking. Filch pounded open the door. Suddenly all the noise ceased.
"My, what is this here?" Slughorn said, astonished, after a pause.
"This boy here claims to have been invited to your party," Filch sneered.
Draco saw a danger sign. Slughorn of course had not invited him, and he would definitely say so. And Professor Snape, who was unfortunately in the small group as well, could see through any lie. He saw no sign of Hermione....
"Alright, alright, I was gate-crashing! Happy?" Draco scoured the office for any sign of....
Slughorn had just allowed him to stay when Snape called him away. He, of course, knew why he was here.