Just wondering if flying home to San Francisco from Rome is better on a Sunday or a Monday?
By "better" do you mean "cheaper"? I don't know, but you can use a website like Google Flights to check that out.
If you want to be at work, on time, on Monday morning, flying on Sunday is better.
Norwegian has a non-stop flight. In the summer, they fly Monday, Wednesday and Friday.
Sunday is the 'day of rest' so winds should be calmer and the flight smoother on Sunday.
However if you fly on Monday, you will have had one more day in Rome.
Your decision to choose what is 'better'
According to this, it's better on a Sunday:
https://youtu.be/_k2hM8N7khk