The holiday originated in the early 1860s, when the New York State Legislature authorized Anniversary Day as a school holiday in Brooklyn, although banks remained open. It was first celebrated on May 28, 1861, on the 32nd anniversary of the founding of the Sunday School Union, a powerful Protestant organization. The law set the first Thursday in June as the day for future holidays.
In 1898, when the City of Brooklyn was consolidated into the City of Greater New York, the city's Board of Education would not recognize Anniversay Day. After vociferous protest, the Board reversed its decision in 1902. A local law reaffirmed the holiday, to celebrate "'the founding of the Sunday school movement in Brooklyn," in 1905. The name of the holiday was later changed to Brooklyn Day, and then, in 1958, because of the number of people who moved from Brooklyn to Queens, it was changed again to Brooklyn-Queens Day, and the day of the celebration was moved to the weekend.
Over time, the origin of the holiday as the celebration of a religious organization has been forgotten, and it has become a secular holiday in celebration of Brooklyn and Queens, despite the complaints of the other boroughs which have no similar day devoted to them. Either way public school ids have the day off and really don't care why. Have fun! We are off to the library's Summer Reading Kick Off party.