Calendars determine the working and non-working days of Tasks in your plan.
Every task has a calendar assigned so that the Gantt can accurately reflect the days that work will and won’t take place.
If you're a Project Admin, you can select the non-working days of an existing calendar or create a new one from the Calendars page of the side-menu.
The last step is to assign the Tasks you’d like to have the calendar from the Gantt.
If you edit a calendar that is already assigned to tasks, all Tasks with this calendar will not be automatically updated.
The updated calendar will need to be re-assigned for the task to reflect the new working and non-working days.