I don't know how young people do things nowadays, and maybe you've already done it, but would I have missed an appointment, I would send a little mail as an apology. This may also help making it seem much less an issue (which it should not be, we all have missed an appointment here and there).
Also don't rely on other people to help you remember stuff. Use an alarm clock, or that phone you probably own.
My spouse and I try to remind the other whatever we have going on but we also both use our own alarms. Why? It helps to avoid ever having any bad feeling toward the other because of a missed whatever. Sure, the other may not reminded us but, first and foremost, it is still us who forgot about it ;)