Rurouni Kenshin: Haru ni Sakura

The "Kenshin Kaden" basically refers to the last extra chapter (comes after the end of the manga and after Yahiko No Sakabatou) of Rurouni Kenshin entitled "Cherry Blossoms in the Spring Time" (Haru Ni Sakura).
It was published in the Kenshin Kaden Artbook.
(Source: The-OrO.com)