The The Devil Is a Part-Timer!, Vol. 15 (light novel) — Satoshi Wagahara

It's the middle of winter, and the Devil King is training in the hopes of transitioning from part-timer to full-timer. However, his plan hits a road block when Emi's mother, Laila, informs Maou and the whole group about a crisis affecting all of Ente Isla. And his full-time job isn't the only thing that might be put on hold-Emeralda and Chiho are planning to throw a Christmas party for Alas Ramus's first winter holiday experience! On top of that, Rika's feelings for Ashiya remain, giving him pause about leaving Japan, and Urushihara couldn't care less about the past of the angels and the tragedy of the heavens. Are the visitors from another world really going to leave Japan for good?!