Fedora 19, recently dubbed Schrödinger's Cat, may overlap a little with Fedora 18 as the current targeted release date is late May, 2013.
To get things straight, Fedora 18 (Spherical Cow) hasn't been released and a Beta version was made available after seven delays.
Now it seems that the Fedora developers are already thinking about the next version and want Fedora 19 (Schrödinger's Cat) out for the public, in final version, on May 21, 2013.
If the past is any indication, that is not going to happen, but it's nice to see them try. A Fedora 19 Schedule proposal ticket has been opened on their website and states the following: “The idea is to overlap F18/F19 development to allow us target May as a release month for odd releases and minimize the effect on development.”
It's not realistic, but a detailed release plan can be found on fedorahosted.org.