According to the National Day Calendar: 

"Every year, Pluto Day on February 18th celebrates the discovery of the former planet. Up until 2006, the dwarf planet was considered one of the nine planets in the solar system.

"Clyde Tombaugh, an American astronomer, discovered Pluto. He made the discovery at the Lowell Observatory on February 18th, 1930 84 years after the discovery of Neptune. Pluto was named for the Roman god of the Underworld.

"The name also honors Percival Lowell. His initials are the first two letters of Pluto. Lowell was an American astronomer who believed there was another planet beyond Neptune. He initiated the search that led to the discovery of Pluto. On March 19, 1930, Lowell Observatory officially announced the discovery of a 9th planet."

https://www.nationaldaycalendar.com/national-day/pluto-day-february-17#:~:text=Every%20year%2C%20Pluto%20Day%20on,an%20American%20astronomer%2C%20discovered%20Pluto.