Install anacron. It'll make sure the daily, weekly, and monthly cron jobs get run when needed. If your Mac was sleeping or off when they were supposed to run, then it'll run 'em shortly after waking up.
These are maintenance tasks that OSX uses to keep itself running smoothly. They are usually run automatically at 3am (I think) if the computer is on 24/7. I think it's a legacy of the UNIX/ BSD system that OSX is built on - someone, no doubt, will correct me or add more info . To do it manually try macjanitor.
There are lots of different freeware app to be found at www.versiontracker.com that do this job. I use MaintainyouMac but I couldn't find it the other day. I have also used Macjanitor which is very good.