Kashō replaces Jōwa
Kashō (嘉祥), also known as Kajō, was a Japanese era name (年号, nengō, "year name") after Jōwa and before Ninju.  This period spanned the years from June 848 through April 851.  The reigning emperors were Ninmyō-tennō (仁明天皇) and Montoku-tennō (文徳天皇).Change of era
February 9, 848 Kashō gannen (嘉祥元年): The new era name Kashō (meaning "good augury") was created because a white tortoise was discovered in Bungo province, and it was duly presented to the emperor.  The previous era ended and the new one commenced in Jōwa 15, on the 13th day of the 6th month of 848.