You can set up as many reminders as you like for an event, each firing independently at its own time before the event starts.
Tap the Send reminder row on the Create Event hub.
Toggle reminders on. This is off by default.
Set the timing for your first reminder, choosing from 30 minutes, 1 hour, 3 hours, 24 hours, 2 days, or 1 week before the event.
Tap Add reminder to add another one with its own timing, and repeat as needed.
Tap Save.
Reminders go out to every invited group member, regardless of whether they have responded yet.
Note: If you edit the event's start time after setting reminders, each reminder's send time is automatically recalculated to stay relative to the new start time.
Turning reminders off
Toggling reminders off removes every configured reminder for that event, and none will be sent.
FAQ
What happens to reminders if I cancel the event?
Any reminder that has not yet fired is automatically suppressed when the event is cancelled.
Can 2 reminders be set to the same time?
You can do this, but you will see a warning: "You have two reminders set for the same time," since it likely was not intentional.