It was a grand day to be a ham.
This picture doesn't go with the postcard below, but was probably from an event very much like the one described on the invitation. Another photo of the picnickers (not shown here) gives evidence that their pith helmets were freebies from a radio company called Hallicrafters.
The OM (old man) was to bring his YL (young lady) or XYL (former young lady/wife) with their homemade hats for friendly competition. These abbreviations are called Q signals or Q codes. Q signals are easier to say, or transmit in morse code than the phrase or question they abbreviate. 73 is a common radio sign-off that means "best regards".
Look how hard they tried to find the adressee. This little postcard went all around Northern Indiana for just a penny stamp. Sadly the postmarks indicate it may have arrived after the event. Let's hope the radio waves brought the details of the day to W9FSA so that he was able to attend, and that his XYL, W9PEX, brought home a big prize for her festive radio-themed hat.