The Twenty-First Street Mission was a homeless shelter established and ran by Edith Keeler in New York City during the early 1930s, in an era known as the Great Depression. At the mission, homeless men and women who had been deeply affected by the economic slump were provided with food and shelter, in exchange for the great care they were given by Miss Keeler, they had to listen to her inspirational speeches about humanity's future.
Shortly after arriving in January 1930 from 2267 via the Guardian of Forever, Captain James T. Kirk and Spock found themselves hiding in the basement of the mission after they were forced to run from a police officer. Instead of turning Kirk and Spock in to the authorities, Keeler took pity on the two runaways and enlisted their help at the mission, whether it be sweeping up, performing general maintenance, or dising out meals.
In March of that year, Doctor Leonard McCoy also arrived in New York City and was experiencing a cordrazine-induced paranoia. When McCoy stumbled into the mission looking mere inches from death, Keeler offered McCoy the use of the cot in her office, where he could recover. After recovering from his condition, McCoy was soon reunited with Kirk and Spock on the front-steps of the mission. Curious of how these three men knew each other, Keeler began to cross the street towards them and was mown down by a large truck, dying on impact. With Keeler's death, her plans for a peace movement died with her, and meant that history proceeded as it should have. (TOS episode: "The City on the Edge of Forever"; TOS - Crucible novel: Provenance of Shadows)
In the alternate timeline accidentally created by McCoy in which Keeler survived, McCoy lived in the mission until March 1931. He expected his colleagues to follow him back in time and rescue him and believed that they would be able to find him more easily if he remained close to the area in which he had arrived in the past. (TOS - Crucible novel: Provenance of Shadows)