Why did the girl invite Uncle Smith to dinner?

W: Good morning, Uncle Smith.
M: Good morning, Cathy.
W: I want to have a dinner party to celebrate my birthday. Would you come?
M: I’d love to, Cathy. What time?
W: Next weekend, on Saturday.
M: Saturday do you say?
W: If that’ s all right with you.
M: I’ m afraid I’m busy then. I’ ve already had plans.
W: What about Sunday?
M: I’ m sure it will be all right.
W: Good, you’ d better come around 6:30 or 7：00, we’ll have time to chat a while.