Project Calendar

What it defines

A project calendar defines which days are working days and which are not. It combines a weekly pattern such as a five-day week with exception dates: public holidays, a company anniversary, site shutdown days. The usual structure is hierarchical — a global calendar shared by the organization, overridden per project where needed. On a project with overseas partners, differing holiday systems show up precisely as calendar differences.

Why it matters

Every piece of schedule arithmetic runs on this calendar. The finish of a 10-day task is not the start date plus 10 — it is the date reached by counting 10 working days from the start. A holiday cluster in between pushes the finish out. The starts, finishes, and float that the Critical Path Method computes are all in working days, so a wrong calendar makes the whole schedule quietly wrong. One missing exception date shifts every downstream task by a day.

Common misconceptions

Confusing calendar days with working days is the most common error. A spreadsheet schedule that computes finish as start plus duration treats every weekend and holiday as a working day. Over a few weeks the drift is invisible; over months it can exceed two weeks. The second trap is treating the calendar as set-once. Holidays change year to year and substitute holidays appear, so on a long project the exception dates need periodic topping up to stay true.