The airline considers red-eyes the last flight of the day and will show them as that. You searched for returns on Saturday the 15th and the red-eye was listed even though it actually leaves after midnight which is Sunday.

Although considered the last flight on Saturday, the note is telling you that it actually operates on Sunday - departing at 12:40 AM Sunday and arriving at 8:11 AM Sunday. That's to let you know not to show up at 12:40 AM Saturday morning.

Most people who make mistakes booking red-eye flights do the opposite. They want the red-eye for Saturday, like you, but see the departure time and think that's the Sunday red-eye so they book the red-eye that departs at 12:40 AM on Saturday and miss their flight.

Jim