My mom is putting the family dog down tomorrow..what should I do?

We have had halle for almost 15 years now and she can barely stand now to go to the bathroom. My mom is putting the dog to sleep tomorrow and I don't feel like I could take seeing someone kill my dog. My mom says that I should be there because they are putting down a "member of the pack" but i think i would rather say my goodbye's today and not tomorrow. what should I do?