On the morning of the event, everyone who said yes gets one message: what it is, what time, and where, with a map link.
It goes at 9 am in the event's own time zone. Nobody who declined gets one, and nobody who never answered gets one.
It is on by default because it is the message hosts say is worth the most — it is the difference between someone remembering at four o'clock and someone remembering at seven. Turn it off for a particular event in that event's options.
You can read the exact wording before the day, and edit it, in the same place.