The compound adjective 'thirty-day' is a modern English formation combining the cardinal number 'thirty' with the noun 'day'. 'Thirty' originates from Old English þrītiġ, meaning 'three tens', a derivative of the ancient root for 'three'. 'Day' comes from the Old English dæġ, a common Germanic term for the 24-hour cycle. The compound follows a straightforward English pattern of number-noun combination to create a specific temporal descriptor, indicating a duration or characteristic tied to a thirty-day period. Its logic is purely compositional, directly linking the quantity to the unit of time.

Structure: thirty (number) + day (unit of time) - thirty: From Old English þrītiġ, related to þrīe (three). A cardinal number, ten times three. - day: From Old English dæġ, of Germanic origin, related to the period of light between sunrise and sunset, or a full 24-hour cycle.