On the Project Calendar Tab you can see all the issues of the current project and you can schedule them here.

Also you you can define working hours of the project here. This will affect the display custom work hours and weekends in the calendar.

 So, these settings will apply to the Issue Calendar Tab (see additional information below).

On the edit settings of project calendar screen you can specify:

• output data (based on the project, favorites filters or JQL-query)
• working time
• weekend showing
• need taskbar 
• need to display the unscheduled tasks 
• ability to display modes: Month, Week, Day
• set the default display mode
• ability to show the avatar and username on issues panels calendar