How to get a file updated server side at given times

I'm not sure where to start with this one, so any help pointing me in the right direction is appreciated.

What I have is a php script saving an XML file from an ftp site to the server (on demand) for parsing/displaying. I've been working with fairly small XML files so this has worked quite well.

Now I have a new vendor whose XML files are over 5MB, so fetching them on demand isn't really an option.

What I want to know is if there is a way to get the server to fetch the file at certain times (twice a day), regardless if the site has been hit to use the data.

Like I said, kind of in the dark on this, so any advice/suggestions/direction is appreciated.

Thanks in advance, -Ted