was a after Kenchō and before Shōka. This period spanned the years from October 1256 to March 1257.Nussbaum, Louis-Frédéric. (2005). "Kōgen" in Japan encyclopedia, p. 542; n.b., Louis-Frédéric is pseudonym of Louis-Frédéric Nussbaum, see Deutsche Nationalbibliothek Authority File . The reigning emperor was .Titsingh, Isaac. (1834). Annales des empereurs du Japon, pp. 248-253; Varley, H. Paul. (1980). Jinnō Shōtōki. p. 231-232.
Change of era 1256 : The new era name was created to mark an event or a number of events. The previous era ended and a new one commenced in Kenchō 8.
Events of the Kōgen era September 1, 1256 (Kōgen 1, 11th day of the 8th month): Kujō Yoritsune, also known as Fujiwara Yoritsune, died at the age of 39 years.Titsingh, p. 252. October 14, 1256 (Kōgen 1, 24th day of the 9th month): Yoritsune's son and successor as Kamakura shōgun, Kujō Yoritsugu, also known as Fujiwara Yoritsugu, died at the age of 18 years.
Notes
References Nussbaum, Louis-Frédéric and Käthe Roth. (2005). Japan encyclopedia. Cambridge: Harvard University Press. ; OCLC 58053128 Titsingh, Isaac. (1834). Nihon Odai Ichiran; ou, Annales des empereurs du Japon. Paris: Royal Asiatic Society, Oriental Translation Fund of Great Britain and Ireland. OCLC 5850691 Varley, H. Paul. (1980). A Chronicle of Gods and Sovereigns: Jinnō Shōtōki of Kitabatake Chikafusa. New York: Columbia University Press. ; OCLC 6042764
External links National Diet Library, "The Japanese Calendar" -- historical overview plus illustrative images from library's collection