From save button you should read user current selection. How you get selected value from combobox depends on how you bind/add data to combobox data. One way like
var name = comboBox1.SelectedItem.ToString();
var day = comboBox2.SelectedItem.ToString();
After getting user selection from combobox you just query to database using ado.net/enterprise library api. Query like
SELECT Count(*) FROM YourTable WHERE EmployeeName = @name AND DayName = @day;
need to execute in database and receive the count result If this count > 0 then the employee and dayname exists. Then you need to show validation message otherwise save the selection.