Going to Longqing gorges

I want to go to Longqing Gorges next saturday and I want to know if i can buy now the tickets for an S2 Subway to go to Yanqing county or I have to want the day of my departure. I want also to come back to BJ the same day, what is the last train time? The ticket from Yanqing to BJ can be purchased also at BJ North Station before going to Yanqing?

Thank you so much for your answers