How to schedule for every two fridays with cron and/or at?

[Kenneth Porter]

--On Friday, July 16, 2004 7:29 PM -0400 Sam Varshavchik 
<mrsam at courier-mta.com> wrote:

> Run the following script every Friday:
>
> if test -f /etc/foo
> then
>      rm -f /etc/foo
> else
>      touch /etc/foo
>      exit 0
> fi

I use this in my weekly backup to alternate which remote directory to save 
to:

# alternate between dump directories each week
# argument evaluates to 0 or 1
/usr/local/sbin/backup-smb $((`/bin/date +%-U` % 2))

The 0 or 1 is used as part of the directory name in my backup-smb script.