Pistol Patent Day takes place on February 25, 2018. When distinguished as a subset of handguns, a pistol is a handgun with a chamber that is integral with the barrel, such as a pepperbox revolver—as opposed to a standard revolver, wherein the chamber is separate from the barrel as a revolving cylinder.
The pistol originates in the 16th century, when early handguns were produced in Europe. Samuel Colt made the mass production of the revolver commercially viable. He applied for his US patent for a "revolving gun" in 1836 he was granted the patent on February 25, 1836. That's why Pistol Patent Day is celebrated an February 25. (With material from: Wikipedia)