Help converting my Script

So i have been asked to write a script for a friend who has a server running solaris 11. My problem is the correct syntax i can't seem to get it work with the follow code, i have read through the date man page and found no work around
Below is the code for part of the rsync script.
The code i need to convert is `date --date=yesterday +\%Y-\%m-\%d`-`date --date=yesterday +\%A` this tells rsync to look at yesterdays date but doesn't work in solaris

#!/bin/sh
rsync -avz /mnt/backups/`date --date=yesterday +\%Y-\%m-\%d`-`date --date=yesterday +\%A`/server1/ /mnt/backups/`date +\%Y-\%m-\%d`-`date +\%A`/server1/

Make sure that the date command is the GNU date command in rsync snippet posted above or specify the full path to GNU date command.

I am currently testing that part of the code and can not confirm that it is working