upon is a preposition that can be used to indicate when something began or when something happened in relation to a certain point in time. It is usually used when discussing events that either started in the past and are still ongoing, or that have already ended. It can also be used to mean 'immediately following', as in 'upon his arrival'.
1. Her studies began upon her graduation from university.
2. Upon the announcement of their engagement, they were met with congratulations from their families.
3. The following day, the police arrived upon receiving a distress call.
Do not confuse the use of 'upon' with 'after', as they typically have different meanings. 'Upon' is usually used to signify a more instantaneous response, while 'after' usually implies more of a waiting period. 'Upon' should also not be interchangeable with 'on', which pertains to dates or parts of a particular day.