Correct application of the Critical Path Method requires consideration of ALL constraints. A calendar is a ‘type of constraint’ that defines when work can and cannot be performed.

There are two main calendar types Activity Calendar and Resource Calendar.

Activity Calendar defines when work technically could be performed.

• A low noise restriction
• Production Freeze period 
• SteerCo calendar

Resource calendar defines the availability of the assigned resource.

Part-time specialist is available 9 am – 5 pm Monday – Wednesday.

Let’s review a simple example with one activity and one resource only:

If a two-day activity could only be performed on weekends (Saturday & Sunday) and the assigned resource only works Fridays & Saturdays, this activity only could be performed on Saturdays. The required effort is two days, activity duration is eight days (Saturday – Saturday).

Not all scheduling tools can simulate scenarios with different activity and resource calendars correctly.

Microsoft Project correctly calculates the duration, start and finish dates (Saturday to Saturday). However, in the activity Gantt chart, it is unclear on which days the work is planned (only by Saturdays, not Sunday to Friday).

Spider Project correctly calculates duration, Start and Finish dates (Saturday to Saturday). Additionally, detailed scheduling in the activity Gannt chart shows exact dates (if needed, time as well) when the task is planned to be executed (Saturday + Saturday).

Primavera also allows having activities and resource calendars. However, it is impossible to calculate the schedule with both calendars simultaneously.
If the activity type is specified as “Task Dependent”, the schedule incorrectly shows that the activity could be performed on Sunday and Saturday:

If the Activity type is specified as “Resource Dependent”, the schedule incorrectly shows that the activity could be performed on Saturday and Friday.