Pluto Day is celebrated annually on February 18 to commemorate the discovery of Pluto in 1930 by American astronomer Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. The day honors Pluto's unique characteristics, including its icy mountains and small size. Initially classified as one of the nine planets in the solar system, Pluto's status was changed to that of a 'dwarf planet' by the International Astronomical Union in 2006 due to its inability to meet the criteria for full-sized planets. The name "Pluto" was inspired by the Roman god of the Underworld and is believed to honor Percival Lowell, whose theories about planets beyond Neptune contributed to its discovery. Typical customs for Pluto Day include educational activities that promote appreciation and awareness of astronomy and the history of planetary discovery.