Dictionary uponTime of an event

preposition

upon

At the time of or immediately after something.

Upon hearing the news, she calls her family immediately.

Etymology

upOld Englishup

+onOld Englishon

The spatial compound acquired a temporal use through immediate association. Upon an event means at its occurrence or directly afterward.

More examples

Please check the contents upon arrival.
He smiled upon seeing his old friend.
The fee is due upon completion of the work.

Forms

upon

Similar words

when: Does not always imply the same immediacy.
after: Does not necessarily imply immediate succession.