If you have a calendar program, such as Outlook, on your computer or device, click this link and follow the instructions.
If you are using an online calendar such as Google or Yahoo Calendar, follow the instructions for your specific calendar and insert this link where instructed
The following help pages were available as of December 2015
Google Calendar Instructions
Apple Calendar Instructions
If neither of those help pages apply to you, try a Google search on how to add an online calendar to "your calendar program".