I just found answer in the internet Here: http://ell.stackexchange.com/questions/1135/difference-between-to-occur-and-to-happen Happen — something random to take place; Occur — something to become observable; in a certain moment; an event or a process; The difference is even more evident if you notice that occur is relative to current as they both came from Latin currere "to run". So, I would use occur in the following contexts: Formal conversations; When a time or place of an event is specific; Expected events, like dates: The Spring Bank Holiday that would usually have occurred at the end of May; Discoveries, ideas: The discovery occurred in Mono Lake, California; ...and happen in the following contexts: Informal context; Rough time or place; Random events, usually without being planned; maybe other native speakers will offer anothe rexplanation)

Each word has different meanings. In the context in which the words are synonyms, the slight difference is that "happen" suggests a degree of suddenness and absence of planning.