Wikipedia writes about the history of the day:
The idea of the Day was launched in 2004. The date of May 17th was chosen to commemorate the World Health Organization’s decision to remove homosexuality from the list of mental disorders in 1990.You'll find more information on dayagainsthomophobia.org.
By May 17th 2005, as a result of a year long campaigning effort, 24 000 people worldwide and reputed international organizations [...] had signed the appeal to support the IDAHO initiative.
In May 2005 already, activities took place on IDAHO in many countries in the world. First LGBT events ever were organized in Congo, China and Bulgaria.
Organizations in more than 70 countries in the world now invest International Day Against Homophobia and Transphobia as part of their annual mobilization plan. In some of them, the Day has become the major focal point or action.
IDAHO has now been officially recognized by the EU Parliament, Spain, Belgium, the UK, Mexico, Costa Rica, the Netherlands, France, Luxemburg and lately Brazil. It is also recognized by numerous local authorities across the world, like the province of Quebec, the city of Buenos Aires, etc.
In several countries like Argentina, Bolivia, Australia, Croatia, etc., national civil society coalitions have called upon by their authorities to have the Day recognized.