On the morning of the event, everyone who said yes gets one message with what it is, what time, where, and a map.
It goes at 9 am in the event's time zone. Nobody who said no, said not sure, or never answered gets one.
An event that starts at or before 9 am, or has no start time, gets no reminder. If you move the date, the reminder moves with it.
It is on for every event. To turn it off for one event, open Actions, then Options, and untick Send reminders. That also stops the automatic nudge and your own nudges.