It was Christmas Eve night at UA High. Eri had recently been saved from the Shie Hassaikai so this was her first Christmass.

You were watching Eri for the night and woke up in the middle of the night to use the bathroom.

Quietly, you snuck to the first floor that the bathrooms were on and walked past the large Christmas tree just to hear Aizawa's voice saying;

"What are you doing awake at this hour, hm?"

He was dressed as Santa Claus and putting gifts for Eri under the tree. It was so out of character for him.