A key date is the rarest, most valuable, and most collected issue within a numismatic series — typically the result of low original mintage, heavy circulation, or extensive melts. Identifying the key date(s) of a series is essential to building a complete collection, as the key date often represents a substantial portion of the total cost to complete the set. Notable key dates include the 1893-S Morgan Dollar, 1916-D Mercury Dime, 1909-S VDB Lincoln Cent, and 1932-D / 1932-S Washington Quarter.

Key date refers to the year, mint mark, or variety within a coin series that has the lowest mintage or the highest survival-adjusted scarcity, and therefore commands the highest price among standard issues of the series. Every long-running coin series has its key dates: 1909-S VDB Lincoln cent, 1916-D Mercury dime, 1932-D and 1932-S Washington quarters, 1893-S Morgan dollar — all are key dates within their respective series.

Key date status drives collector behaviour: many series collectors complete the rest of the series first and save the key date for last, which sustains demand and price for decades. Key-date premiums are particularly steep at higher grades, where surviving population thins quickly. Counterfeiters target key dates more aggressively than common dates, so authentication and grading become especially important at this tier. See our 1916-D Mercury dime entry for a worked example of a classic key date.