@johny It's archaic. Nobody plans that far ahead any more. https://en.m.wiktionary.org/wiki/overmorrow
@johny I was just trying to come up with a joke reason it might have fallen out of use. I like the sound of the word Ubermorgen a lot better than Overmorrow, myself. Might have been as simple as a historical tonal preference for "the day after tomorrow"
@nobody Overmorrow sounds like literal translation from many languages, how do you mean nobody plans 2 days ahead? You must overdrink on Friday so you can recover on Saturday on Sunday so you can be ready for work on Monday. Weekend drinkers do that all the time.