Keiō (慶応?, historically 慶應) was a Japanese era name (年号,, nengō,?, lit. "year name") after Genji and before Meiji. The period spanned the years from April 1865 to September 1868. The reigning emperors were Kōmei-tennō (孝明天皇?) and Meiji-tennō (明治天皇?).
Read more about Keiō: Change of Era, Events of The Keiō Era