International Asteroid Day is an annual event which is held on the anniversary of the Siberian Tunguska event that took place on June 30th1908. It is unclear whether the explosion was caused by a comet or an asteroid, but it flattened 22,000 square kilometers of forest.
Asteroid Day was founded 2014 by filmmaker Grigorij Richters, Apollo 9 astronaut Rusty Schweickart, Queen guitarist and astrophysicist, Brian May, and the B612 Foundation, a NGO dedicated to the defense of the Earth against asteroids. Over 200 astronauts, scientists, technologists and artists signed the Asteroid Day Declaration.
Learn more at NASA asteroid fun facts.