Girlfriends simply get annoyed at this because it seems to them that their boyfriends don't care about the relationship as much as they do. It's the little things that matter sometimes and special days are very important little things. Girlfriends feel that (and I realize that not ALL girls are like this) if their boyfriends can't remember these important days that mean so much to her then he simply doesn't care that much about the her or the relationship as much as she does. And to know that your significant other doesn't care as much as you do is the same as having a crush on someone and realizing they don't like you back. It hurts. So guys, next time if you forget an important date and your girlfriend gets annoyed, just take the blame and apologize, she is simply just a girl in love. =)