Kagen
Kagen (嘉元) was a Japanese era name (年号,, nengō,, lit. "year name") after Kengen and before Tokuji. This period spanned the years from August 1303 through December 1306. The reigning emperor was Go-Nijō-tennō (後二条天皇).
Kagen (嘉元) was a Japanese era name (年号,, nengō,, lit. "year name") after Kengen and before Tokuji.  This period spanned the years from August 1303 through December 1306. The reigning emperor was Go-Nijō-tennō (後二条天皇).

Change of era
1303 Kagen gannen (嘉元元年): The new era name was created to mark an event or series of events. The previous era ended and the new one commenced in Kengen 2.

Events of the Kagen era
July 17–27, 1303 (Kagen 1, 13th-23rd days of the 6th month): A white comet ("broom star") was seen at azimuth in the northeast each day at dawn for 10 days.
October 4, 1305 (Kagen 3, 15th days of the 9th month): Former Emperor Kameyama's death.

Notes
References
Nussbaum, Louis-Frédéric and Käthe Roth. (2005). Japan encyclopedia. Cambridge: Harvard University Press. ISBN 978-0-674-01753-5; OCLC 58053128
Pankenier, David W., Zhentao Xu and Yaotiao Jiang. (2008). Archaeoastronomy in East Asia: Historical Observational Records of Comets and Meteor Showers from China, Japan, and Korea. Amherst, New York: Cambria Press. ISBN 9781604975871 ISBN 1604975873; OCLC 269455845
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. ISBN 9780231049405; OCLC 6042764

External links
National Diet Library, "The Japanese Calendar" -- historical overview plus illustrative images from library's collection