The rather terrible cold would give her a ... well, a rather terrible cold.

“That’s the Blacks’ Annual Ball,” Sirius pointed out, cheerfully.

She tore herself away from her thoughts, and realized Sirius was speaking. “Of course we can miss it,” he was saying, rather impatiently. “There’s nothing special about that ball, Nic. So why don’t you just stay over for the night, and we can do something fun like cook –”