Calendar for JIRA is a common JIRA plugin. So it could be installed like the other plugin through Add-ons.
For start using the plugin you must:
Create Two Custom Fields which can be of two types - "Date " (Date Picker) and "Date / Time " (Date Time Picker) with any name to store the information about the start date and end date of the issueses.
Do not use the same fields (start and finish) to work with the plugin Gannt-Chart for JIRA.