Adding holidays to Calendar

It is possible to add public holidays to your N900 calendar by importing an appropriate file in iCalendar (.ics) format.

Go to the Holidays page on Mozilla, or any other site from which you can download an iCalendar file with the data you require.
Select the link for the nation of your choice.
Select Open with Calendar.
Decide whether to add to an existing calendar or create a new one; the latter allows you to delete en bloc and replace with more up-to-date versions should that become necessary).
Observe your calendar populated with holidays.
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Profit!