You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello. How can I help you?

USER: I am Debra Green, I have to attend my graduation ceremony on 10/20 in San Francisco, CA - SFO. can you please book my ticket?

ASSISTANT: May I know the source and return day?

USER: I want to mention that, I have to be there one day before the event. I want to start from Washington DC - DCA and my return day is on 10/21. My depart day is on 10/19.

ASSISTANT: Sorry to tell that there is no flight found on your request.

USER: OK, thanks for your service.

ASSISTANT: Thank you for contacting us.