According to the Team USA site, more than 160 nations participate through formal Olympic Day events. The International Olympic Committee page for the festivities offers some details.
"Encourage people to get active on Olympic Day. 'Move' can refer to all sorts of physical activity for people of all ages and abilities."
Their use of the word 'move' reminded me of instructions from a famous Nick who appeared frequently on Saturday Night Live.

Olympic bid cities are getting in on the action, too, with LA2024 hosting Olympic Day festivities in Los Angeles Memorial Coliseum.
"Olympic Day is a unique opportunity to get a taste of what it means to inspire the next generation around the positive and powerful values of sport and Olympism," said LA24 Vice Chair and Director of Athlete Relations Janet Evans.
Image via IOC