Nagato Yuki-chan no Shoushitsu: Toaru Ichinichi (とある一日, The Disappearance of Nagato Yuki-chan: A Certain Day) is a third-party light novel based on The Disappearance of Nagato Yuki-chan.
It is a collection of short stories based on the events of volumes 1 and 2 of the manga. It is sold by Kadokawa Sneaker. It contains 36 chapters.