Event is the subject and occurred is the verb
The probability based on an event that has already occurred is 100%. If the event has occurred, it has occurred.
The constitutional convention occurred second.
The Vietnam War is the event that occurred in the Tonkin Gulf.
September 15, 1963 The Birmingham Church Bombing. Killed 4 little girls that were there for Sunday School.
September 25th 1963 was a Wednesday.
September 18, 1963 fell on a Wednesday.
The probability of event A occurring given event B has occurred is an example of conditional probability.
No memorable space event occurred on this day.
What important event occurred April 9th, 1865
What important event occurred April 14th, 1865
The event occurred on the date specified.