anacron (periodic command scheduler)

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.

Every time Anacron is run, it reads a configuration file that
specifies the jobs Anacron controls, and their periods in days.  If a
job wasn't executed in the last n days, where n is the period of that
job, Anacron executes it.  Anacron then records the date in a special
timestamp file that it keeps for each job, so it can know when to run
it again.  When all the executed commands terminate, Anacron exits.

It is recommended to run Anacron from the system boot-scripts. For 
example, add the following lines to /etc/rc.d/rc.local:

  if [ -x /usr/sbin/anacron -a -f /etc/anacrontab ]; then
    /usr/sbin/anacron -s > /dev/null 2>&1
  fi

This way the jobs "whose time has come" will be run shortly after the
machine boots.  A delay can be specified for each job so that the
machine isn't overloaded at boot time.

In addition to running Anacron from the boot-scripts, it is also
recommended to schedule it as a daily cron-job (usually at an early
morning hour), so that if the machine is kept running for a night,
jobs for the next day will still be executed.

See anacron (8), anacrontab (5) and /usr/doc/anacron-2.3/README
for further information.