Consider the following variant sentences:
Colloquia and seminars both happen in an academic setting.
Colloquia and seminars both occur in an academic setting.
Using happen to describe ongoing events often sounds very wrong to my ear — I usually prefer occur or some such variant, but I have no idea why that preference should be justified. Is using occur (or something like that) actually better here, or am I being overly pedantic? I’d love an explanation.
Occurs invokes the concept of a definite start or beginning. It sounds better to you because it usually gets at a more precise meaning of the concept that you want to communicate (based on your examples).
“The sound always occurs at midnight.”
Happens is very similar but does not invoke the concept of the events start or beginning.
It is subtle, but happens would be more vague in the way that you used it.
Occur is more formal, and while it is generally fully interchangeable with happen, it may be more specific in implying the time or action of an event.