Celebration (party)

A celebration is a kind of party on the occasion of either something joyous that is happening or has just happened:

a birth, etc., see below
passing an examination
a success in sport, business, etc.
having got a better or own home
having got a (better) job
being released from hospital, prison, kidnapping

or commemorating something joyous that happened in the past, e.g. a whole or round number of years ago, and for which the consequences still continue, e.g.:

birthday if the person is still alive
wedding if the partners are still together
a country became independent or liberated, which is still the case

The celebration may also be on the occasion of a special day in general, e.g. New Year.