Can either mean
Immersed in a certain topic
more than usual for a long period of time.
Having trouble concentrating on your surroundings and
daydreaming more than usual.
Named after Gundolf Köhler, who was known for being
extremely immersed in Nazi Germany and was told to be living in his own fantasy world by his friends.
Guy 1: Hey man, why'd you get all these games out?
Guy 2: I guess it's a Gundolf day.
Guy 1: Ah, I see.
Guy 1:
Hey dude, are you
listening to me?
Guy 2: Uh, what? Sorry, i wasn't paying
attention
Guy 1: Gundolf Day?
Guy 2: Haha, yeah.