How obsolete is the word “overmorrow”?
Is there a one-word English term for the day after tomorrow? Perhaps a term that has fallen out of modern English usage.
One that would complete the sequence of: today, tomorrow, ...
There may have been one, or more, and there may still be dialectal variants around here and there. But there's no general word; instead there's a fixed phrase, which you used: the day after tomorrow.
Germanic languages can use the word for morning to refer to the next daybreak. In German Morgen still means both morning and tomorrow; in English morrow, a variant of morning, came to be used in the latter sense. The to- is probably a fossilized definite article.
In German, with its transparent morphology, there is a word Übermorgen that means the day after tomorrow, but English is morphologically naked. If there were such a word, it would be overmorrow.