Originally proclaimed by the UN General Assembly in 1993 it has been organised annually on 3 May by UNESCO.
World Press Freedom Day serves as an occasion to inform citizens of violations of press freedom: in dozens of countries around the world publications are censored, fined, suspended and closed down, while journalists, editors and publishers are harassed, attacked, detained and even murdered. It is a date to encourage and develop initiatives in favour of press freedom and to assess the state of press freedom worldwide. It also serves as a reminder to governments of the need to respect their commitment to press freedom.
There's more on the UNESCO website at www.unesco.org/new/en/unesco/events/prizes-and-celebrations/celebrations/international-days/world-press-freedom-day/.