What is a Project Calendar?
The project calendar defines when project work may happen — working days, shifts, holidays, seasonal windows — while resource calendars overlay when specific people and equipment are available. Schedules computed without the right calendars are arithmetic fictions: five "days" that span a national holiday week aren't five days.
Multiple calendars routinely coexist: office staff on five-day weeks, site crews on six, permit-restricted night work on its own, and a marine works window that only exists four months a year.
Worked example
A bridge project's calendars: general works six days/week; in-river piling restricted to the July–October environmental window; deliveries banned during the town's festival week. The scheduler loads all three — and the software immediately shows that missing this year's river window costs not two weeks but nine months. That single calendar constraint reorganizes the entire early sequence, exactly as it should.