Our friends over at Wikipedia explain the meaning of National Aviation Day:
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The holiday was established in 1939 by Franklin Delano Roosevelt, who issued a presidential proclamation which designated the anniversary of Orville Wright's birthday to be National Aviation Day (Mr. Wright, born in 1871, was still alive when the proclamation was first issued, and would live another nine years). The proclamation was codified (USC 36:I:A:1:118), and it allows the sitting US President to proclaim August 19 as National Aviation Day each year, if desired. Their proclamation may direct all federal buildings and installations to fly the US flag on that day, and may encourage citizens to observe the day with activities that promote interest in aviation."