I am going though the Dev Doc of activiti where I found one post regarding The "business calendar" is used when calculating due dates for tasks.(URL: https://docs.alfresco.com/activiti/docs/dev-guide/1.4.0/#_business_calendar ).I have done with the changes mentioned in docs its working fine when u set the due date in the format of - Fixed Duration after task creation for ex.skip Saturdays and Sundays. So, if a task is created on Friday with a 3 day due date, the due date would be the following Wednesday. But when you use the Expression definition due date format it doesn't work. please let me know weather I need to do some more changes in class or I am missing something.
