Help needed for school planner application

I am developing a school planner application and I am struggling to solve this issue. Essentially, the app asks users to input their lessons for each day which is then stored in one long list (under the variable name timetable). I have successfully made it so that users can see their schedule for the current day by sorting through the list to get the lessons for that particular day. However, I would like to add a button that would allow the user to see their schedule for the next day. It seems that the logic required to do this is quite complex and I have tried many different things to no avail. Could someone suggest a possible way to do this.

Below is what I have so far, thanks in advance.