postmorgaŭ

Two words you already have. Post is after. Morgaŭ is tomorrow. Stick them together and you get the day after tomorrow, in one word, with no new root to learn. English needs four words for this and German needs one (übermorgen); Esperanto builds it rather than borrowing it, which is why it is a nice small demonstration of what the language is for.

And it has a mirror. Antaŭhieraŭ is the day before yesterday, built the same way from antaŭ (before) and hieraŭ (yesterday). The pair is exactly symmetric, and learning them together costs nothing and fixes both:

• antaŭhieraŭ - the day before yesterday
• hieraŭ - yesterday
• hodiaŭ - today
• morgaŭ - tomorrow
• postmorgaŭ - the day after tomorrow

Five days in a row, and only three of them are roots. Antaŭhieraŭ arrives a few ranks after this word in the same frequency band, so the course teaches them almost together.

Notice the -aŭ. Hieraŭ, hodiaŭ and morgaŭ all end in -aŭ, and so do almenaŭ, preskaŭ and ankoraŭ. That ending marks a small closed set of primary adverbs that Zamenhof gave a distinct shape so they could not be mistaken for nouns or adjectives; it is not a suffix you can attach to anything. The aŭ is one syllable, so postmorgaŭ has three: post-mor-gaŭ.

Using it. As an adverb it needs no preposition: postmorgaŭ ni veturos. The adjective postmorgaŭa exists for the day's own things - la postmorgaŭa kunveno, the meeting the day after tomorrow - and the accusative of time, postmorgaŭan tagon, is possible but nobody writes it. For the rest of the clock and calendar, see telling the time and dates.