My boss thought that this is a great idea and since we're dealing with mental health, this is one way to express emotions. 

How would you celebrate this day?

There is a site called that explains how the day came about. From the site:

πŸ™‹ Why July 17?

July 17 is famously displayed on the iOS Calendar Emoji, which is why we chose it as the date for World Emoji Day.
Even if you are using a system with a different calendar emoji, that shouldn’t stop you joining in this global celebration of emojis! The date is part the reason this day was chosen, but the day is much bigger than just one emoji.