Hello Ilona Yeromina
How can I buy it, because it says that all the tickets have already been bought. Please help me.
If all Saver Day Passes are sold out, then no such pass is available for purchase 🙂
If you hold the Swiss Half Fare Card / Half Fare Card, you could purchase a Day Pass for the Half Fare Card at CHF 75 (you must be in the possession of this card to purchase the ticket):
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/day-pass-for-the-half-fare-travelcard.html
Otherwise, there are no national public transit day passes, which can be purchased spontaneously.
If you want to benefit from Saver Day Passes, you have to plan and purchase them in advance.
Kind regards,
Roland