Difference between Happen and Occur

HAPPEN vs OCCUR: VERB

To come to pass.
To come about as a consequence; result.
To be the fate; become.
To encounter something by chance.
To come or go casually; make an appearance.
Used with the impersonal subject it and a following clause or infinitive to indicate the occurrence of a usually unexpected or chance event.
To have the fortune (to be or do something); be in a situation or position (to be or do something).
Used with an infinitive to make polite requests.
To come by chance; to come without previous expectation; to fall out.
To take place; to occur.
To meet with; to fall or light upon.
To make a casual call.

To meet; to clash.
To come to the mind; to arise in the mind; to suggest itself; to be presented to the imagination or memory; -- of an idea or thought.
To happen; to take place.
To go in order to meet; to make reply.
To come to mind.
To meet one's eye; to be found or met with; to present itself; to appear.
To be found to exist or appear.
To take place; come about. : happen.

Possibly; perhaps.
To occur by chance; occur unexpectedly or unaccountably; in general, to occur; take place.
To chance; be by chance or unexpectedly: as, he happened to be at home.
Fortunate; happy; blessed.
(Santayana)
Come into being
Become reality
Come upon, as if by accident
Meet with

To run to, as for the purpose of assisting.
To run together; meet; clash.
To strike the senses; be found; be met with: as, silver often occurs native; the statement occurs repeatedly.
To emerge as an event into the actual world; happen; take place; come to pass; befall: as, what has occurred?
To strike the mind: with to.
Suggest itself
Come to one's mind
Synonyms To come to pass, come about, fall out.
To refer: with to.
Eccles., to coincide in time, so as to interfere each with the celebration of the other: as, two holy days occur.

Come about, Pass off, Bechance, Go on, Take place, Hap, Encounter, Chance, Pass, Bump, Hit, Find, Befall, Materialize, Occur

Affect, Erupt, Ensue, Recur, Arise, Come about, Go on, Pass off, Pass, Take place, Hap, Be, Come, Anticipate, Happen

Recur, Unfold, Come, Come about, Go on, Take place, Hap, Encounter, Chance, Pass, Bump, Hit, Find, Materialize, Occur

Materialize, Exist, Affect, Erupt, Ensue, Recur, Arise, Go on, Come about, Pass, Take place, Hap, Come, Anticipate, Happen

It basically describes all the horrible things that happen in hell and all the awesome things that happen in heaven.
Make sure it is clear what will happen when the rules are followed and what will happen when they are not.
And if it can happen to Jennifer Love Hewitt, it can happen to you.
Richard Stamp of London had one of the worst things that can happen to a man happen to him, he lost his fucking penis.
Although this used to happen regularly under the Supporting People programme, it does not happen regularly now.
And that again will not happen immediately, but could happen over the next year or so if his conviction is upheld.
That way at least you know what will happen should something similar happen in the future.
Meaning: If you want something to happen, make it happen.
Some want it to happen, some wish it would happen, other make it happen.
Yet assumptions have a major impact on what individuals expect will happen, can happen, or should happen.

Wars occur when sovereign nations challenge each other and wars do not occur when formerly warring parties are incorporated into a larger political unit.
This issue is more likely to occur where the employer is not a registered health practitioner, but can occur with any employer.
Although not every activity may occur for every migration, the steps that occur should generally follow the sequence shownbelow.
Should this occur at a population level, the effect on the emissions that occur within the City of Melbourne woulddramatic.
Surveillance can occur at any point in time and can often occur around your scheduled IME appointment times.
Most often, the problems occur during pregnancy; however, they may also occur during childbirth or shortly after birth.
Harassment, bullying, aggression and other bad behaviors occur when the culture facilitates the opportunity for them to occur.
Serious side effects can occur during treatment with this medicine and can sometimes occur without warning.
Atomicity ensures that either all actions occur or none occur.
Some environmental changes occur slowly, and others occur rapidly.