I can't give a big contribute, but I wanted to write this post to "spread the word".
I'll link you here the official website.
From the official site:
"May 17th is symbolic due to its significance in the improvement of the status of gays and lesbians. IDAHO proposed this date for annually holding a day set aside to fight homophobia. In removing homosexuality from its list of mental illnesses on a May 17, the World Health Organisation (WHO) put an end to over a century of homophobia in the medical field.
Borne out of the International Conference on LGBT Community Human Rights held in Montréal from July 26 to 29, 2006, prior to the opening of the first World Outgames, the Declaration of Montréal included a recommendation to have an International Day Against Homophobia on May 17 of each year recognised."
Today there was a flashmob against homophobia in Rome, but i live too far to the capital so I couldn't partecipate.