Social and political significance:
The park has historically been the start or the end point for many political demonstrations. In April 1861, soon after the fall of Fort Sumter, it was the site of a patriotic rally of perhaps a quarter of a million people that is thought to have been the largest public gathering in North America up to that time. In the summer of 1864 the north side of the square was the site of a "Sanitary Fair".
Union Square is, and was, a frequent gathering point for radicals of all stripes to make speeches or demonstrate. In 1865 the recently formed Irish republican Fenian Brotherhood came out publicly and rented Dr. John Moffat's brownstone rowhouse at 32 East 17th Street, next to the Everett House hotel facing the north side of the square, for the capitol of the government-in-exile they declared. On September 5, 1882, in the first Labor Day celebration, a crowd of at least 10,000 workers paraded up Broadway and filed past the reviewing stand at Union Square. Although the park was known for its union rallies and for the large 1861 gathering in support of Union troops, it was actually named for its location at the "union" of Bloomingdale Road (now Broadway) and Eastern Post Road (now extinct) decades before these gatherings. On March 28, 1908 an anarchist set off a bomb in Union Square which only killed himself and another man.
Union Square was named a National Historic Landmark in 1997, primarily to honor it as the site of the first Labor Day parade.[
In the days and weeks following the September 11, 2001 terrorist attacks, Union Square became a primary public gathering point for mourners. People created spontaneous candle and photograph memorials in the park and vigils were held to honor the victims. This was a natural role for the Square as Lower Manhattan below 14th Street, which forms Union Square's southern border, briefly became a "frozen zone," with no non-emergency vehicles allowed and pedestrians sometimes stopped and asked why they were venturing south by police and national guardsmen. In fact, for the first few days following the attacks, only those who could prove residency below 14th Street could pass. The Square's tradition as a meeting place in times of upheaval was also a factor.