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These are synonyms, so I use them equally
The only slight variation is that Occur is often meaning To come to mind, while Happen is more To take place
happen is closely associated with blind chance. Things that “happen” can’t really be anticipated or guarded against.
Occur is a bit more definite as in this warning on a bottle of an Antibiotic: “Discontinue use and consult your doctor if any adverse reactions occur."
there is no difference between them
Occur mean available or living but used for not surety and Happen use for available with surety.