3 May 1991
According to UNESCO (United Nations Educational, Scientific and Cultural Organisation), the objective of World Press Freedom Day is “to evaluate press freedom around the world, to defend the media from attacks on their independence and to pay tribute to journalists who have lost their lives in the exercise of their profession.” World Press Freedom Day was declared on 3 May 1991 and the day has been observed since.
References

MDDA, Celebrating World Press Freedom Day, from Bizcommunity, [online], Available at www.bizcommunity.com [Accessed: 31 March 2014]|UNESCO, About World Press Freedom Day, from United Nations Educational, Scientific and Cultural Organisation, [online], Available at www.unesco.org [Accessed: 31 March 2014]