I don't have enough context to write an accurate overview. You've indicated the context is "year," but "21" could refer to multiple things associated with years or the number 21 (such as the card-counting film, a book, an age of adulthood, etc.). Could you clarify what "21" refers to?
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AD 21 (XXI) was a common year starting on Wednesday of the Julian calendar. At the time, it was known as the Year of the Consulship of Tiberius and Drusus (or, less frequently, year 774 Ab urbe condita). The denomination AD 21 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).