Where is my crontab file on OS X?

Where is it located?

Cron tabs

/etc/crontab
/usr/lib/cron/tabs/*

Source Mac OS X 10.9 - Artifacts Location

Cron tabs are now stored in /var/at/tabs, which is a protected directory. You will need to change to a root shell in order to view them.

They can be viewed by typing

sudo su; cd /var/at/tabs; ls

which will ask you for your password. You will notice that you can see the cron tab for your current user as well as the cron tab for the root user inside of this directory. Once you are done backing up your cron tabs make sure to type exit to leave the root shell so that you don't accidentally do something with escalated privileges :)