First started in 1999 by the creator of the smiley, Harvey Ball, the holiday celebrates the intent behind the now universally recognized icon - to bring a smile of the faces of people around the world. The purpose of the holiday was the move away from the commercialization of the smiley and to use it as a symbol of kindness and affection.
Morale Booster

The smiley made its debut in 1963 as a way to increase the morale of the employees of a life insurance company. Today, it is one of the most popular nonverbal ways to convey joy, happiness and cheer. The smiley face began as a bright yellow circle with two dots to represent eyes and a black line that represents a smiling mouth.
Emoji

Today, there are many different versions of the smiley, representing many other human emotions such a sadness, surprise, worry and laughter. In modern texting smileys are used to express emotions in online conversations on a smartphone or on a computer. The practice has its origin in Japan, and the smileys and other icons are known as emoji.
How to Celebrate?

Make people around you smile.

Do a random act of kindness. Buy a stranger a coffee. Compliment someone. Give your seat up to someone on the bus.
Surprise someone in your family with a visit, cook a meal for them, or do something special for them.
Volunteer your time and/ or money at a local nonprofit organization.

Did You Know...

...that babies are born with the ability to smile?

More: timeanddate.com