A project calendar or a project planning calendar is a tool to help you organize a project's timeline. It lets you plan project milestones and ensure you deliver things on time. But a planning management calendar is just one of the four calendars involved in project management. It should be noted here that, in general, project management calendars are used for three key major purposes: scheduling tasks, managing resources, and delivering. A scheduling task lets you align one task after another in a chain of events. At the same time, managing resources is also crucial for a project plan because a certain team, key executive, or any other resources may only be available at certain times.