City Fading to Silver (S04E01)

Illya's fun summer break comes to a sudden end. The aftermath of their battle against the Heroic Spirit of the 8th Class Card disappeared overnight from Mt. Enzou. Their suspicions aroused, Illya and the others race to the scene, where they encounter two mysterious women who say they've come to retrieve Miyu... After a fierce battle, they're all bathed in a mysterious light. Afterwards, Illya wakes up to a midwinter Fuyuki City?! (Crunchyroll)