Cron is great for servers, but for your dev box (be it (Power|i)Book), which may be switched off or sleeping for long periods of the day, you really need anacron. More info here. Install once, then just put symlinks to tasks in,

/etc/periodic/daily
/etc/periodic/weekly
/etc/periodic/monthly

If you want more (cron-like) power over execution intervals and conditions, you'll need to create a launchd plist file. Lingon provides an interface.