As expressed in our History of the IAUP: The First 30 Years (Vol. I of the IAUP History, p.107) the establishment of the worldwide UN International Day of Peace, was unanimously approved by the UN General Assembly during its 36th Session, on November 30, 1981. The UN designated the third Tuesday of every September as the yearly Day of Peace.
The UN also designated the year 1986 as the UN International Year of Peace, just as IAUP had requested as well. This occurred during the 37th UN General Assembly Session on November 16, 1982, and was declared officially at the 40th Anniversary of the UN on October 24, 1985.
The IAUP proposal for the establishment of both the Day of Peace and the Year of Peace arrived to the United Nations through a proposal coming out of a resolution drafted during its 1981 Triennial Conference in San José, Costa Rica (June 28- July 3, 1981.) A letter had been sent by Costa Rican Ambassador and Permanent Representative Rodolfo Piza Escalante to the Secretary General of the United Nations, at the request of the IAUP. Dr. Young Seek Choue, President of Kyung Hee University, Korea, did very active lobbying directly in New York, prior to this result.
A copy of the resolution to establish the UN International Day of Peace can be seen in the following link: http://www.un.org/documents/ga/res/36/a36r067.htm
The official letter sent by Ambassador Piza can be seen in the following link: https://disarmament-library.un.org/UNODA/Library.nsf/ddd51e79a30bc9d28525757d004e225a/6034f33c44cf7ca88525761c006159c8/$FILE/A-36-197.pdf