I'm going to cancel a job interview the morning of it
Why do you want to cancel the interview?
It will only waste my time.
If you know this far in advance that it'd be a waste of your time, then you should cancel it now.  Waiting until the last minute to cancel is a very unprofessional way to handle your affairs.  It may make it harder to interview there again in the future should you change your mind.
I don't think I need to explain myself to you. It's my decision.
It's your life, I'm just telling you that canceling interviews last minute can give you a really bad reputation.