Hi eli3
The Saver Day Pass is valid as far as Wengen only. See the map below.
From Wengen you’d need to buy a ticket at half fare given you are holding the (Swiss) Half Fare Card. Supersaver tickets are not available for touristic trains.
It is a fact that Jungfraujoch is a very pricey mountain trip. Alternatively, check for the Top of Europe pass if you stay multiple days in the area.
It is advisable to buy the tickets at the day of travel only, if you can be certain that the weather will be good. On a rainy day there is zero point in going to Jungfraujoch.
Best
Roland
https://www.sbb.ch/content/dam/internet/sbb/de/abos-billette/abonnemente/karte-geltungsbereich-ga-halbtax-stand_d_barrierefrei.pdf.sbbdownload.pdf