cron-like program that doesn't go by time

 Anacron (like `anac(h)ronistic') is a periodic command scheduler. It
 executes commands at intervals specified in days. Unlike cron, it
 does not assume that the system is running continuously. It can
 therefore be used to control the execution of daily, weekly and
 monthly jobs (or anything with a period of n days), on systems that
 don't run 24 hours a day. When installed and configured properly,
 Anacron will make sure that the commands are run at the specified
 intervals as closely as machine-uptime permits.
 .
 This package is pre-configured to execute the daily jobs of the Debian
 system. You should install this program if your system isn't powered on
 24 hours a day to make sure the maintenance jobs of other Debian packages
 are executed each day.