Why did Cathy invite Uncle Smith to dinner?

W: Good afternoon, Uncle Smith.
M: Good afternoon, Cathy.
W: There will be a dinner party for my birthday. Would you come?
M: I’d love to. When?
W: Next Saturday.
M: I’m afraid I’m busy then. I have to meet a foreign friend at the airport.
W: That’s OK. What about Sunday?
M: That will be all right.
W: That’s great. You can come at around 7:00 and we can chat for a while before the party starts.