How does a happening differ from an activity

An activity is something people do, a happening is just an event in which something happens. For example, an activity could be a sport while a happening could be your birthday

A happening refers to something that just occurs and an activity is something specific, a task, someone is doing.

Explanation:

A happening can be any event that just takes place, a theatrical performance, for example. An activity is something an individual carries out, it is easily identifiable as a task which usually takes up time and resources.