What exactly is going to happen at the Climate Signing Ceremony on 22 April?

 Why is this signing ceremony being held on 22 April?

The Paris Agreement calls for the Agreement to be open for signature from 22 April, 2016 to 21 April, 2017. To mark the first day that the Agreement will be open for signing (which coincides with Earth Day), United Nations Secretary-General Ban Ki-moon invited leaders from all countries to attend a high-level signing ceremony to accelerate climate action and to drive the political momentum towards ratification and early entry into force of the Paris Agreement.

Didn’t countries already adopt the Paris Agreement in Paris? Didn’t they agree to everything?...

http://www.un.org/sustainabledevelopment/blog/2016/04/what-exactly-is-go...