Hello Kctong
You must buy a day pass for Zurich valid on the 16th. It is not necessary to buy it in advance, such day passes are also available at the airport (vending machines or ticket office).
Chose a return ticket Zurich Airport - Zürich HB for a train leaving the airport at 2pm, it will be automatically converted to a 24h-pass.
Kind Regards
Danhiel