Are you using data driven pages? If you are then you can use Wes Miller's code in a display expression for your data driven page layer like so:
def FindLabel ( ):
from datetime import date, timedelta
return date.strftime(date.today() - timedelta(days=1), '%m/%d/%Y')
Then in your dynamic text element you would use this:
<dyn type="page" property="expression" />
This works for me and should update automatically for all your maps since it uses the current date code.