A key date is term which is used in coin collecting and in the numismatic world and it refers to a particular date or a specific date and mint combination in a series of coins that were minted during a particular period in a particular country which have over the years become very hard to obtain and thus hold immense value. The key date is used for a coin that is rare and highly desirable raising its value generally.
While the Morgan dollars have over the years become immensely popular and the series has itself become interesting and entertaining to collect, the Morgan silver dollar key dates are amongst the rarest and amongst the most sought of coins amongst US minted coins. They are popular not only because of their composition which comprises of 90% silver and only 10% copper but also because of their rarity outside. This is because many of the Morgan coins never circulated rendering them as uncirculated Morgan dollar coins while many coins were also melted during World War I to meet demands for silver. This makes the Morgan dollar coin extremely hard to find and very rare to come across. Amongst the series, the rarest are the CC minted Morgan coins, which were minted at Carson City for a short period of time making the number of coins issued relatively less thus making the coins rare and precious. More generally speaking, key dates are very highly collectible and manage to not only maintain but also very significantly raise their values over the years.
Amongst all the Morgan coins, the most desirable remains to be the 1895 (Philadelphia) proofs. According to records, there were around 12000 coins that were struck in 1895 at the Philadelphia mint but there remain in existence no samples or examples of this coin. But there are 880 proofs of the Morgan dollar at the Philadelphia mint. However, of these 880, only around 75- 80 have been accounted for to date.
More general key dates for the Morgan dollar are: 1883-S (uncirculated grades are scarce), 1884-S (uncirculated grades are scarce), 1892-S (uncirculated grades are scarce), 1893-S, 1894-P, 1895-P, 1895-O, 1895-S, 1896-S, 1899-P, 1901-P (uncirculated grades are scarce) 1903-O, 1903-S and the 1904-S.