Anacron 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.
WWW: http://anacron.sourceforge.net/
from Andreas Kahari <andreas.kahari@unix.net> with some cleanup by me