Xoogigem97
You can buy tickets on this site:
https://www.gotthard-panorama-express.ch/en/
Saver Day Passes are still available for this day and you can choose them as ticket. They are valid from the border station of Chiasso and offer full flexibility in Switzerland, your can travel with every train, boat or bus on the day of validity, except for mountain excursions, for example they are not valid to Pilatus.
With a Saver Day Pass, you need also tickets from Milan to Chiasso. You can buy them on sbb.ch. If you choose an Eurocity train, please note that tickets are valid only in the selected train in Italy. In regional trains tickets are valid for 4 hours.