Always on the 19th, so the actual day will vary. Here is a quote from one of the holiday origin websites:
"National Aviation Day celebrates the myriad of amazing contributions, inventions, and developments leading to and resulting from human flight. On December 17, 1903 Wilbur and Orville Wright were able to launch their airplane and watch it fly for a momentous twelve seconds and a total distance of 120 feet. Though this may not seem remarkable in light of the extensive aviation industry that has since flourished, that brief but historic flight made all the innovations of modern day air travel and navigation possible. The Wright brothers constructed the airplane that undertook the flight by hand, with the successful design approximately four years of intensive research and trial and error in the making. National Aviation Day was established as such by presidential proclamation in 1939, which designated the anniversary of Orville Wright's birthday on this date in 1871 an annual holiday to mark the effort and dedication to the idea of human flight, without which the world as we know it would be a very different place."
