The concept of the project calendar refers specifically to the particular calendar in which the project is outlined and or delineated via a few specific mechanisms. One key element is that the project calendar features the working days or shift whose inclusion on the calendar firmly establishes the particular dates on which the assorted schedule activities are scheduled to take place and are scheduled to be worked. Equally importantly, the project calendar will carefully outline and delineate the days in which no project schedule activities are scheduled to be worked, days in which those schedule activities are determined to be idle. Those days are of particular importance because those open days are days in which alternate work or activity may be assigned, although in many cases in which projects are tightly scheduled the only idle days may prove to be holidays, weekends, or other predetermined days off. Also see resource calendar.
This term is defined in the 3rd and the 4th edition of the PMBOK.