4/3/2010:
Robert Moon first invented the ZIP code in the 1950s. It was originally a 3-digit code designed for mechanical sorting, which Moon thought would help postal services keep up with the influx of mail after WWII.
However, it took some time for the idea to catch on. Some places began using ZIP codes by 1963, but it wasn't until this date in 1965 that Postmaster Dunnel in Minot urged residents to start using ZIP codes. That was because the Minot Post office was to be equipped with mechanical sorting devices within 18 months. Properly labeling letters and packages allowed for full utilization of the machines.
The change took time, but it added ZIP to the process of sending mail everywhere.
Dakota Datebook written by Sarah Walker
Sources:
The Minot Daily News, April 3, 1965, p.11