National Let's Laugh Day (19 March)

"I can find no reference to this day before 2017, or who first floated the idea...To some extent it's been overtaken by World Laughter Day -- the first Sunday in May -- created in 1998 by Dr.Madan Kataria, who founded the worldwide Laughter Yoga movement..." (Crystal)
There is a science of laughter (gelotology), and its role in conversation is undoubtedly interesting. But who needs another "day" for it? Seeing if you could abstain from laughing for a whole day might be an interesting way to observe it.