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").
Time-based freezes will allow the Debian Project to blend the predictability of time based releases with its well established policy of feature based releases. The new freeze policy will provide better predictability of releases for users of the Debian distribution, and also allow Debian developers to do better long-term planning. A two-year release cycle will give more time for disruptive changes, reducing inconveniences caused for users. Having predictable freezes should also reduce overall freeze time.
We previously discussed talks between Canonical and the Debian release team about fixed freeze dates.