You did not define any method declared in DayOfTheWeek. You are printing monday and tuesday as if you overloaded the ostream << operator ( which you didn't )
Well there are several things you might want to do...differently
First, you declared your methods setDay, printDay, and getDay, and ... never actually implemented the methods?
In this case simply
1 2 3
void DayOfTheWeek::setDay(string instring){
Day = instring;
}
and so on with the rest of the methods.
on another note, you never stored the value of monday.getDay();
on another note again, you can just << monday I believe, if you overload the << operator. But I don't think that's what you were trying to do when you were cout'ing the day's value.