Any user can create an unlimited number of calendars. From the main menu JIRA you should go to the "Calendar" and on the right side click "Add Calendar" then chose "Create New" link.
To tune in your calendar you could go to the "Calendar" on main tab and on the right side click "Calendar's Cog" then chose "Set Up" link.
You can create calendars for different cases: planning specific project or a product, employee, group of employees or the special issue types (such as a vacation or a business trip, or marketing activities).