Using Custom Labels in a Class
You can access these custom labels like this:
if(monitoringFrequency == System.Label.daily){
return dateValue.addDays(1);
}else if(monitoringFrequency == System.Label.weekly){
return dateValue.addDays(7);
}else if(monitoringFrequency == System.Label.monthly){
return dateValue.addMonths(1);
}
Do make sure that the names of the custom labels are correct.
For more information, see: https://help.salesforce.com/HTViewHelpDoc?id=cl_about.htm&language=en_US
We can put label in salesforce these ways:
Visualforce Page:
$Label.Label_API_Name
Lightning Component Page
{!$Label.c.Label_API_Name}
Apex Class
String getLabelName = Label.Label_API_Name;
or directly
System.Label.Label_API_Name
hope this helps you.