I was kinda pissed that Windows Server 2008 R2 Backup GUI had so little scheduling options,
you have to choose a "Once a day" bakcup or "More than once a day".
So to customize a shcedule to suite ones needs, simply create a backup with "Once a day" option, then in the Administrative Tools open Task Scheduler and navigate to:
Task Scheduler Library/Microsoft/Windows/Backup
there you should find your backup job, right click it and select Properties.
Go to Triggers tab, select trigger Daily and press Edit. Here you have more options for scheduling. Good luck.