IDAHO is the "International Day against Homophobia and Transphobia".
IDAHO was founded by Louis Georges Tin in 2005. Campaigns and inititives take place on or around May 17th every year to combat prejudice against LGB-people and transgenders. May 17th is chosen because it marks the anniversary of the day in 1990 (!) when the World Health Organisation removed homosexuality from its list of mental diseases.