You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I help you?

USER: Donald Williams here. I want to attend Christmas party, please book my ticket from ATL to SFO.

ASSISTANT: May I know your desired travel dates?

USER: My travel dates are 12/26 and 12/28.

ASSISTANT: There is no flight on your desired travel dates.

USER: It's ok, thank you.

ASSISTANT: Thank you for opting us.