If you're not officially his girlfriend, I'm thinking maybe you could just let it go. Seriously, guys don't like to be hassled, and neither do they like to talk about every little niggle that goes "wrong"...
if you think you can just take a deep breath and think to yourself "right, well that trial was really important to him, and seeing as we're not that committed, and if this had happened any other day it wouldn't be such a big a deal, and perhaps he is genuinely sorry he couldn't meet up and wants to meet on thursday instead.... it doesn't mean he doesn't like me anymore" then I would recommend you do that.
True, it wouldn't have killed him to mention Valentine's day, but seriously... if you get annoyed about little things like that, that's what's going to drive him away.