There is a hierarchy of celebrations in the Church. Some are mandatory, some are not. Some are worldwide, some are not.
There are, of course, obligatory days. These are Sundays and other days of highest obligation. There are other days that optional such as weekday Masses and certain memorials. Here are some terms to keep in the back of your mind:
SOLEMNITIES: These are the highest celebrations. When celebrated they begin with evening prayer of the previous day. They celebrated important events, persons of highest importance to the faith and even certain beliefs. These appear on the universal calendar.
FEASTS: These celebrations are not quite as high as the solemnities but still of great note.
MEMORIALS: These are more local celebrations. They may be limited to a country or even a single location such as a monastery or church and so are not on the universal calendar. They may or may not be mandatory – most likely not. They can even be optional. Sometimes there are a number of these on one day and one can be picked from the number of them or even skipped altogether.