What Is Schooldays Well Difined Or Not Well Difined

What is schooldays well difined or not well difined

 

A = {School Days}

A = {Monday, Tuesday, Wednesday, Thursday, Friday}

The set is well-defined because it is possible to determine the school days.

If it was something like, "cold days", it would be not well-defined because people have different opinions on how "cold" a day is.