Moving Day March 20th 1941!

In the early days of 1941 the United States had 862 AM stations. Of those 40 were clear channel stations. At the same time Canada had 6 clear channel stations and Mexico had zero. The inequity was obvious, but the channel assignments reflected a technological inequity that dated back to the 1920s. The result was that Mexico refused to regulate their border blasters creating a whole different kind of "clear channel" station which blazed interference across the continent. More here

This was solved by treaty in 1938 with the signing ceremony being held in Havana, Cuba. The hard part would be rearranging the furniture on the US radio dial to make it happen. This was not one project, but many. It was moving day for approximately 776 American AM radio stations. The complexity of the logistics cannot possibly be overstated. In this era of analog electronics it meant new crystals. Further complicating the plan, the FCC decided they should all shift frequencies simultaneously on March 20, 1941.  And they were not alone. 100 Canadian stations, and "numerous" broadcasters in Mexico, Cuba, Haiti and the Dominican Republic would need to shift frequencies as well. The total tally of stations effected by the pact was estimated to be around 800.

In their defenseof the change they only stated that it would "...serve to eliminate in considerable measure the long-complained of interference from these sources, and thereby improve broadcast reception in the North American generally. Interference from Mexican and Cuban stations has been particularly objectionable to the rural listeners." The treaty was known as the North American Regional Broadcasting Agreement (NARBA).

Stations that normally signed off at night did swap out hardware overnight and sign on with new frequencies. Other stations with directional antennas had more work to do. But exactly how many stations moved is uncertain. In the FCC 1941 annual report noted that 802 of the 893 US changed frequencies. The original list had only said 862 stations. Most moves went well but some caused interference and required additional moves and shuffles going on for years. The NARBA delegation continued to meet into the early 1950s to iron out the kinks but the worst was over.