pebedaan occur dengan happen?

Based on freedictionary.com

occur [əˈkɜː]
vb -curs, -curring, -curred (intr)
1. to happen; take place; come about
2. to be found or be present; exist
3. (foll by to) to be realized or thought of (by); suggest itself (to)
[from Latin occurrere to run up to, from ob- to + currere to run]

Usage: It is usually regarded as incorrect to talk of pre-arranged events occurring or happening: the wedding took place (not occurred or happened) in the afternoon

and happen :

hap·pen (hpn)
intr.v. hap·pened, hap·pen·ing, hap·pens
1.a. To come to pass.
b. To come into being.
2. To take place or occur by chance.
3. To come upon something by chance.
4. To come or go casually; make an appearance.