International Moment of Laughter Day is celebrated on April 14, 2017. The goal of this day is to get people to laugh, because "laughter is the best medicine". Moment of Laughter Day was created by Humorologist Izzy Gesell to encourage people to laugh.
Laughing is an involuntary reaction to certain external or internal stimuli. Laughter can arise from such activities as being tickled, or from humorous stories or thoughts. Most commonly, it is considered a visual expression of a number of positive emotional states, such as joy, mirth, happiness, relief, etc.
A link between laughter and healthy function of blood vessels was first reported in 2005 by researchers at the University of Maryland Medical Center with the fact that laughter causes the dilatation of the inner lining of blood vessels, the endothelium, and increases blood flow.
Laughter has also been shown to have beneficial effects on various other aspects of biochemistry. For example, laughter has been shown to lead to reductions in stress hormones such as cortisol and epinephrine. When laughing the brain also releases endorphins that can relieve some physical pain. (With material from: Wikipedia)