When suprised or shocked by something that has been said or that something has been done, such as an event.
Sometimes the "days" is considerably lengthened due to the severity of the suprise e.g "dayyyyyyyyyyyyyyyyyyyyyyyyyyyyys of our lives!!"
"Did u hear that Stan got off with stacie last nite?"
"Days of our lives!"
"Wow, Accrington Stanley just beat Man Utd 9-0!"
"Daaaaaaaaays of our lives!!!"
Can be shortened to just "days!".