Why do They Call It Union Square?

Many people presume that this well-known park got its name from its
proximity to the headquarters of many trade unions. Indeed, the area has
long been associated with labor, as well as grassroots and radical causes.
On the night in 1927 that Nicola Sacco and Bartolomeo Vanzetti ("Anarchist
Bastards," according to the judge who sentenced them) were executed in
Massachusetts, a huge rally was held for them in the Square.

Alas, the real reason for the name is more mundane: it is at the union of
two major north-south thoroughfares: Broadway and 4th (aka Park) Ave.