Just in case you were wondering...
Within the classic US commemorative series, there are several multi-year programs, ranging from two years (e.g., the 1892-93 World's Columbian Exposition) to 14 years (i.e., 1926-39 Oregon Trail Memorial). Typically, the event being commemorated was not a multi-year event, but the respective coin sponsor sought to profit from its coins as much as possible via multiple issues. However, in a couple of cases, coins issued in two consecutive years made historical sense - the 1920 and 1921 Pilgrim Tercentenary Half Dollars are one such pair.
The Pilgrim Tercentenary Half Dollar includes a prominent "1620-1920" dual date within its reverse design, with the "1920" doing double duty representing the coin's terminal anniversary date as well as its issue date. While some may argue that a dual date that serves to establish "bookends" for the anniversary being celebrated by the coin should not simultaneously be used to denote its issue date, it is a common, long-standing practice to do so that continues into the modern era (e.g., the "1776-1976" found on the US quarters, half dollars and dollars struck for the US Bicentennial).
After an Atlantic Ocean crossing of over just over two months, the Pilgrims anchored in the harbor near present-day Provincetown, MA on November 11, 1620. After sending out several search parties to gather fire wood and look for a suitable settlement site, such a site was found across present-day Cape Cod Bay from Provincetown; it is thought that the better location was found on or about December 10. After a week of bad weather, the Pilgrims went ashore at present-day Plymouth, MA and began their settlement efforts on December 18, 1620.
Over the brutal winter that ensued, about half of the colonists died (50 of 102) from exposure, disease and/or starvation. In the Spring of 1621, the Pilgrims signed a peace treaty with the local Wampanoag Native Americans and were subsequently guided by them regarding the planting of crops such as corn, beans, wheat, barley, pumpkins and peas. In the Fall, after their first corn harvest, the Pilgrims organized a meal in celebration and invited some of the Wampanoag to join them (the First Thanksgiving).
Tercentenary celebrations took place in 1920 and 1921 in Provincetown and Plymouth to mark the events of 300 years before, with the largest events held in Plymouth during the Summer of 1921.
The language in the coin's enabling legislation did not limit when the coin could be struck or what year/date could be placed on it - beyond setting a maximum mintage of 300,000 coins, the coin's Act did not include many restrictions/limitations. So, when the Pilgrim Tercentenary Committee ordered more of its authorized coins in 1921, the Mint simply added a small "1921" to the obverse and left the dual dates of the anniversary on the reverse untouched. The "1920" end of the "1620-1920" dual date did, however, lose its issue date significance with the move!
Though the date was placed on the coin to identify the year in which it was struck, as described above, the "1921" date also ties into the early history of the Plymouth Colony. As such, the Mint, by following its general policy regarding dating coins with the year in which they are struck, created a coin with even more historical context than its first-year issue. 1921 Pilgrim Tercentenary Half Dollar
For more of my topics on commemorative coins and medals, including more on the Pilgrim half dollar, see: Commems Collection