The official calendar of the Church is the General Roman Calendar. This calendar dictates official feast days throughout the world. Individual countries can have additions or variations from the General Calendar. On a few feasts you'll see a USA National Calendar date. These are official feasts in the USA on that date.
You'll also see Common Calendar listed for several feasts. These are feasts no longer on the General Roman Calendar but ones that have been celebrated historically.
Finally, coming soon, you'll also see a 1962 Pre-Conciliar Calendar date listed for several feasts. This is the date the feast was observed on the General Roman Calendar prior to the Second Vatican Council.