Task Calendar overrides Project Calendar

Create a calendar for a task within Project By default, tasks (task: An activity that has a beginning and an end. Project plans are made up of tasks.) are scheduled based on the project calendar (project calendar: The base calendar used by a project.). To define unique or specific exceptions (exception: An instance in which a task or resource calendar differs from the project calendar. An example is a resource that must work on Saturdays, but for which the project calendar does not include working time to cover these instances.), such as machinery that runs during nonworking time (nonworking time: Hours or days designated in a resource or project calendar when Project should not schedule tasks because work is not done. Nonworking time can include lunch breaks, weekends, and holidays, for example.), or an office move that can occur only on a weekend, you can create a task calendar (task calendar: The base calendar that you can apply to individual tasks to control their scheduling, usually independent of the project calendar or any assigned resources' calendars. By default, all tasks use the project calendar.) for individual tasks that override the project calendar.
Refer: Setting Calendars
On the Tools menu, click Change Working Time