World Laughter Day was created in 1998 by Dr. Madan Kataria, founder of the worldwide Laughter Yoga movement. It is most often celebrated by gatherings of people in public places with the sole purpose of laughing. Its popularity has grown exponentially with that of the Laughter Yoga movement now counting thousands of Laughter Clubs in more than 105 countries. Now, it is celebrated worldwide.
Here is a video on the benfits of laughter yoga.