How do I run a cron job on a Mac? How can I make my Mac (running the most recent OS) do a task automatically once a day? For example, I'd like to have it automatically download a copy of this open source algebraic geometry textbook once a day. I have a feeling that some combination of wget and cron should work, but, after googling and reading the cron documentation, I just couldn't get it to work right.
I think in OS X you should use launchctl for "cron jobs".
But otherwise your task should be easy to do with a cronjob
sudo crontab -e
55 23 * * * wget http://www.math.columbia.edu/algebraic_geometry/stacks-git/book.pdf  

This should run everyday at 23:55

Update

You should specify the output location of WGET with -O /path/to/file