When are auto-reminder emails sent?

An Auto Reminder is counted from the response's Launch Date, and it arrives at a predictable time of day. Reminders set to send before a completion date work a little differently.

Reminders set a number of days after

You set the timing on the Auto Reminder email template, in Assessment Settings > Emails:

1. The count starts at the Launch Date. If the response has no launch date, it is counted from the date the response was created instead. This matters when a response is scheduled to launch on a future date: the Welcome email goes out when the response launches, so the reminder is timed from that launch date rather than from the day the response was set up.
2. A reminder arrives at about the same time of day as the launch. A response launched at 3:00 PM with a 1 day reminder is sent between 3:00 PM and 4:00 PM the next day.
3. You can pin them all to one hour. Set Reminder Send Time in Site Settings > Email Settings and every "days after" reminder goes out at that hour instead, using your site's timezone. Leaving it on "(Not set - send every hour)" keeps the default behavior.

Reminders set a number of days before completion

1. Reminder Send Time does not apply. These are sent as soon as they come due, whatever hour you have chosen there.
2. They arrive within a window. A reminder set to 1 day before completion is sent at least 24 hours, and up to 47 hours, ahead of the Requested Completion Date. Only the date is used, not the time of day on it.
3. A completion date is required. If the response has no Requested Completion Date, this reminder cannot be sent at all.

For full details, see What time of day are scheduled emails and reports sent? and Choosing the Right Email Template.