It was a surprisingly good day because he didn’t intend Harry to try to rescue Hermione. Harry was supposed to find out about her death later. He couldn’t be absolutely sure her death would have the precisely correct effect on Harry, either. It was surprisingly good because he got Dead Hermione and Dark Harry out of the deal; an expectedly good day would have been just getting the former.
It was a surprisingly good day because he didn’t intend Harry to try to rescue Hermione. Harry was supposed to find out about her death later. He couldn’t be absolutely sure her death would have the precisely correct effect on Harry, either. It was surprisingly good because he got Dead Hermione and Dark Harry out of the deal; an expectedly good day would have been just getting the former.