Problem

Unable to add Calendar Gadget in Jira by following the steps mentioned in this document: Adding the Calendar Gadget

This problem still exists even after you have tried to click Load all gadgets on the right side.

Cause

The JIRA Calendar Plugin is not installed.

Resolution

1. Go to Administration>Add-ons>Find new add-ons
2. Search for "calendar"
3. Install the  "Jira Calendar Plugin" 
4. Once the Plugin is installed , go to Dashboard page and click on the "Add Gadget" button
5. Search for "calendar" in the search box
6. You will not see anything yet , so you have to click on the "Load all gadgets" at the right side
7. Now you may see the "Jira Issue Calendar" available in search result which is on the right side
8. Click on the "Add Gadget" button to add it into the dashboard
9. Close the "Add a gadget" pop-up as it already added into the dashboard in the background
10. Choose any project at the Project or Saved Filter then click Save
11. Now you may see the calendar gadget appear on the dashboard