The compound adjective 'three-day' is a transparent and relatively modern English formation, following the common Germanic pattern of number-noun compounding. Its first element, 'three', descends from the Proto-Indo-European numeral tréyes, a root foundational to counting systems across Indo-European languages, evident in Latin tres and Greek treis. The second element, 'day', originates from the Proto-Germanic dagaz, which likely meant a specific span of daylight or a full cycle. The literal combination, "three days," directly informs its modern grammatical function as an adjective to describe events, periods, or objects with a duration of exactly three days, such as a 'three-day weekend' or a 'three-day pass'. The logic of its meaning remains strictly compositional, with no semantic shift beyond the sum of its parts.

Structure: three (3) + day (24-hour period) - three: From Old English þrīe, þrēo, of Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as a cardinal number. - day: From Old English dæg, of Germanic origin; related to Dutch dag and German Tag. Functions as a noun denoting a 24-hour period.