If you want to enter some exceptional working days in a Merlin project, for example when needing to plan and schedule work during the weekend, you do this in the ‘working time’ inspector.
If you need this information for the whole project, you click the top most entry in the activities views. You switch to the ‘working time’ inspector and add an exception for the specific date in the ‘exceptions’ tab.
To define that this is an exceptional working day, you use the mouse and select the appropriate working intervals for this exception.
BTW: If this exception affects only the work of a specific resource, you add the exception in the calendar of that resource.