Harry and Snape are stuck together on Christmas Eve.

Potential ideas/options:

• They're at Grimmauld Place for a meeting, but Harry has been living there with the Weasleys.  Harry has been quarantined all day because he's been sick and there are so many people there or coming and going, they don't want him to spread it around.  Later that evening, the door opens, and Molly ushers Severus in, also sick, and now also in quarantine.
• Harry has been placed in a safe house for his own protection.  Maybe he's already 18 or 19 and out of school, and has been told to stay put or else.  Snape is brought a few days later, and is told to stay put for his own safety.
• Harry and Snape get locked into a room in the castle on accident and can't get out.
• Lily’s old protection spell draws them both into the same space on Christmas Eve.