advance bullet train booking

Hi,

I am going from Xian to Huashan in the evening by bullet rail on 25.10 around 8-9pm (Sunday) and going back 26.10(Monday) in the evening around 7pm. I have to get to Xianyang airport at approximately 9-10pm.

I am wondering do I need to book in advance those tickets as I cannot afford to miss return trip to XA. How busy is this route on those days and time?

Thank you in advance