The first day of pink was held in September 2007 at a rural high school in Cambridge, Nova Scotia. A grade nine student had worn a pink polo shirt on his first day of school. He was bullied by a group of grade 12 students using homophobic and transphobic slurs. Bullies threatened that if the student ever wore pink again, he'd pay.
17 year old David Shepherd and Travis Price heard about the incident and took action. They bought 50 pink tank tops from a discount store and used social media to organize what they called a "sea of pink" in solidarity against bullying. More than 300 students showed up in pink.
Since then, the day of pink or pink shirt day has spread to schools around the world as a day against bullying. Specifically, the day has been adapted to target LGBTTQI youth who are most at risk for bullying and related suicides. The day is also a chance to raise awareness about bullying and tackle homophobia. It is celebrated on April 11.
To support the day of pink, check out this organizing guide or sign this petition for the Ontario Accepting Schools Act.