Apple's recommended why is launchd. Search for Lingon or Launchd Editor for some GUI tools to help you compose the schedule.
However, cron still works in Leopard and I agree that a simple cron entry is far easier than dealing with launchd, especially since I am put off by some of launchd's earlier (Tiger) limitations.
Apple removed all the cron templates in Leopard but the cron scheduler is still there and works fine. To get cron going in OS X, you need to either create a file called /etc/crontab or create user cron tabs in /var/cron/tabs. Since you are likely the only user and you need only one entry, simply use /etc/crontab.
The syntax is simple, each value separated by a tab:
30 18 * * * peterf /path/to/script
This means:
30 is the minute (0 - 59)
18 is the hour (0 - 23)
* is the day of the month (1 - 31)
* is the month of the year (1 - 12)
* is the day of the week (0 - 6, 0 being Sunday)
peterf is the user that you want to run the script
/path/to/script is the full UNIX path to the script, case is sensitive
That is pretty much it. The file must be a plain text file with UNIX line breaks, so use a command line editor like vi, pico, or emacs to create. Once the crontab file is present cron should kick off and start working. If it does not, then you may need to use launchd to start cron. This can be done with:
sudo launchctl -w load /System/Library/LaunchDaemons/com.vix.cron.plist
You likely will not need that however.
