Why can’t the woman miss the dentist appointment?

M: So, Susan, do you have anything planned for this Saturday?
W: Uh, I’m kind of busy. Why do you ask?
M: Oh, I was wondering if you like to get together and do something, like catch a movie or take a walk down by the lake.
W: I’d love to, but I’m really going to be busy all day on Saturday.
M: What do you have going on that day?
W: First, my mom asked me to help clean the house in the morning, and then I have a dentist appointment at 12:30. I can’t miss that because I’ve canceled twice before.
M: Well, what about after that?
W: Well, I’m going to be running around all day. After the dentist appointment, I need to meet Julie at 2:00 to help her with her science project that’s due on Monday morning at school.
M: Okay, but are you free ’after that?
W: Hardly. Then I have to pick my brother up from soccer practice at 4:30, and my mom asked me to cook dinner for the family at 5: 30. I feel like a slave sometimes. Then, I have to clean the dishes and finish reading my history assignment. Who knows how long that’ll take.
M: Wow, sounds like you’re going to have a full day. Hey listen, why don’t I come over later in the evening, and we can make some coffee and watch a movie.
W: Oh, that’d be great, but our video machine is broken.
M: Huh. Well, let’s just play a game or something.
W: Sounds good, but give me a call before you come. My mom might try to come up with something else for me to do.