They're somewhat similar and can be used in interchangeable in some context but if I were to use i'd prefer to use them the way I explained above -Happening is rarely used as a noun when considered the frequency of others. So it's usually used like in this example “The events happening recently has devastated us.” you see it's not a noun it's a reduction it is used instead of “that has been happening” -Occasion can be used interchangeable with event from time to time but it also can mean your job besides that, it expresses a situation rather than an “event” that takes place so while “On rare occasions, I like to wear my red dress.” is much more natural than “On rare events, I like to wear my red dress.” Even though you probably would be understood that way too “The event tonight will be flawless.” cannot be expressed as “The occasion tonight will be flawless.” because in that example we are talking about a something that takes place at somewhere such as party or wedding etc. So event is often used for something more concrete. -Experience is something, well you experience so it's not very interchange. - Occasion and case are very similar that they're almost always interchangeable except case being used in law or occasion used as profession. On some rare cases, the disease can reoccur and On some rare occasions the disease can reoccur seem similar but has a different in fact I don't know if it's true but from what I've seen so far case is used for more serious context like the one I've given above but occasion is more personal like the example I give about occasion while explaining it.