Days.Sun = (int) 10;//error here
Think about what you are trying to do here...
"Days" is the enum and "Sun" is one of the enumeration values... Completly confusing...
Do you like mabye to do something like this?
int day= (int)Days.Sun
Anyway what do you expect from the value '10' from an enum in the range 0..6?
Ok, on the other hand in case you like/need to have a specific relation for your enum you can do something like this
enum Days { Sun= 4, Mon= 5, Tue= 6, Wed= 7, Thu= 8, Fri=9, Sat=10 };