It’s proven quite tricky to find something suitable. But I did find some data on another website, hopefully it contains International Days which are valid for the whole world. Here is the data imported into a Google calendar.
[googleapps domain=”www” dir=”calendar/embed” query=”mode=agenda&src=5tricr65360di2192q2v95j9bk%40group.calendar.google.com&ctz=Europe/London” width=”640″ height=”600″ /]
You may also be interested in downloading the official UN App (Calendar Of Observances Making a Difference) for your phone or tablet. This allows you to view and import dates directly into your phones calendar, you can download the free app directly from iTunes.