jdk 8 does not allow string in switch statement ?? why

error:
Cannot switch on a value of type String for source level below 1.7. Only convertible int values or enum constants are permitted code.  

Here is an example:

String typeOfDay;
         switch (dayOfWeekArg) {
             case "Monday":
                 typeOfDay = "Start of work week";
                 break;
             case "Tuesday":
             case "Wednesday":
             case "Thursday":
                 typeOfDay = "Midweek";
                 break;
             case "Friday":
                 typeOfDay = "End of work week";
                 break;
             case "Saturday":
             case "Sunday":
                 typeOfDay = "Weekend";
                 break;
             default:
                 throw new IllegalArgumentException("Invalid day of the week: " + dayOfWeekArg);
         }