Excursion discount available on non-travel days?

Hello,

When I would like to use a 25% discount (for Piz Gloria for example), as a holder of the Interrail pass, is it required to go on this excursion on one of my travel days? Or may this excursion be planned on an no-travel day as well (still receiving the discount)? We are staying close to the start of an excursion and it would help not using 1 of our travel days when planning our trip to the Piz Gloria :).

Thanks a lot in advance!

Best regards, Max