The date of March 20 was chosen to celebrate the anniversary of the Agency for Cultural and Technical Cooperation (ACCT) which was born on March 20, 1970, in Niamey, Niger. 
This agency marks the first step towards Francophone cooperation, which will become the International Organization of La Francophonie a little later. (UNESCO https://www.unesco.org/en/days/francophonie)
The United Nations Educational, Scientific and Cultural Organization (UNESCO) and the International Organization of La Francophonie (OIF) share this common vision that places culture and language at the heart of any lasting effort for peace. and development.