from what I hear, not very crowded. I also believe that is a Get a friend in free day w/ your season pass. I've never been there on opening day so Im prob. not much help.
I think CoasterDude316 posted something about his trip to SFGAm Mother's Day last year. I believe he stated it was one of his best trips. I've had friends go on Mother's Day in years past, and they didn't face too many crowds.