Who should say sorry?

Haha funnily enough I'm in exactly the same position as you right now. Totally forgot my friend is having a birthday thing next week, and I have a dance class that night which I have to go to because my exam is soon. Now she's being completely moody with me. Best thing to do is apologise, say it how it is and let it go. Nothing more that you can do, because you're being honest with her. After a few weeks, it'll all be fine again.