Prior to this date, new versions of packages will be automatically imported from Debian testing where they have not been customized for Ubuntu, that is when the version number of the package in the current Ubuntu development branch does not contain the substring "ubuntu" and there is a new and tested version in Debian. The import is done by copying the source package verbatim from Debian and building fresh binary packages on the Ubuntu autobuilders.
By this date, all packages that have been modified by both Debian and Ubuntu since the last merge should have been merged at least once in the current release cycle.
After this date, packages will only be imported from Debian testing in this way on a case by case basis or by explicit request from a developer. (If the package needs to be modified for Ubuntu, or is not in Debian, then a developer can of course also upload it directly.)