Today it means: September 30th; A national observance in remembrance of the time of year that Indigenous children were forcibly removed from Native homes to be forced into schools to assimilate into the colonizer's culture and lose their birth culture; and the cultural genocide and deaths of children at the schools, caused by the residential school system. Observers frequently wear orange shirts to stand in solidarity with survivors of the residential schools. First observed in 2013. Full definition →
From orange + shirt + day. From the colour of the shirt worn by a survivor of the residential school system, which was forcibly removed by Christian missionaries, who desired to assimilate indigenous children forcibly enrolled at the schools.
Etymology from Wiktionary, CC BY-SA 4.0.