I understand that Milan has restricted areas during the week. However, we are planning to visit Milan on Sat and Sun.
Am I correct is assuming that ZTL will not be a problem those days?
Thank you

There are multiple ZTLs in Milan, and they have different hours. I know there's at least one (Navigli) that operates seven days a week. There are also streets where only certain types of vehicle can operate, and days on which only certain classes of (less polluting) car can circulate. It's very complex.

We have friends who live in the center of Milan. They have monthly bus passes, and use their car mostly for trips out of town. When we visit them, we take the train.

Only Monday to Friday. See https://en.wikipedia.org/wiki/Milan_Area_C

That's not the only ZTL.

The Navigli ZTL is much smaller than the Centro Storico one (Area C). I found a map, but I can't get a working link.