The antonyms for "thirty-first" are words that describe everything except that specific day, such as "previously," "later," or "not applicable." Other antonyms include "second," "third," and "fourth," which all represent different, consecutive dates of the month. Additionally, words like "beginning," "middle," or "end" do not refer to any specific day but instead describe a general range of time. "Inconclusive," "indeterminate," and "indefinite" also represent antonyms as they do not provide a concrete date or time frame. Similarly, words like "ambiguous," "unclear," and "vague" are antonyms because they describe something obscure, uncertain, or hard to define.