http://www.markshuttleworth.com/archives/1252
Mark Shuttleworth announced it on his blog on 13.04's release day.
Also in
https://launchpad.net/ubuntu/saucy milestones have been created to implement the monthly snapshot concept that was argued for in the last UDS, to potentially do away with the alpha and beta milestones.
Ubuntu 13.05 - Released May 31
Ubuntu 13.06 - Released June 31
Ubuntu 13.07 - Released July 31
Ubuntu 13.08 - Released August 31
Ubuntu 13.09 - Released September 31
Ubuntu 13.10 - Released October 31
and if this theme were to continue into the "T" series it would be as follows
Ubuntu 13.11 - Released November 31
Ubuntu 13.12 - Released December 31
Ubuntu 14.01 - Released January 31
Ubuntu 14.02 - Released February 31
Ubuntu 14.03 - Released March 31
Ubuntu 14.04 LTS - Released April 31
and it would roll on and on...