From the Ubuntu's Mother dept.:
The Debian project has decided to adopt a new policy of time-based development freezes for future releases, on a two-year cycle. Freezes will from now on happen in the December of every odd year, which means that releases will from now on happen sometime in the first half of every even year. To that effect the next freeze will happen in December 2009, with a release expected in spring 2010. The project chose December as a suitable freeze date since spring releases proved successful for the releases of Debian GNU/Linux 4.0 (codenamed "Etch") and Debian GNU/Linux 5.0 ("Lenny").