San Juan County, New Mexico was established on February 24, 1887 and named for the San Juan River (itself named for St. John the Baptist by Spanish explorer, Juan Rivera, in 1765)! Its county seat is Aztec, and its fair is held in Farmington! You might recognize the New Mexico part of Four Corners, Shiprock, Salmon Ruins, Chaco Culture National Historical Park, and Aztec Ruins National Monument, which are located in this county!