Hello ana sanchez
It probably would have been more convenient to purchase an EC ticket from Zurich to Milan instead of a Saver Day Pass. The latter is only valid as far as Chiasso (and not to Milan).
With the Saver Day Pass, you either have to purchase an additonal EC ticket from Chiasso to Milan or an Arcobaleno ticket from Chiasso to Milan. You can get it at www.sbb.ch/en.
The Arcobaleno ticket is not valid on EC trains; you have to change trains in Lugano, can continue on the RE train with your Saver Day Pass to Chiasso, from where your Arcobaleno ticket is valid.
A seat reservation is not mandatory on Swiss trains and only possible on IC trains.
Kind regards,
Roland