Last day of the Julian Calendar in Catholic countries. The next day, the Gregorian calendar took effect in Italy, Poland, Portugal, and Spain by the order of Pope Gregory XIII. The calendar was put in place to realign events like equinoxes and solstices with the calendar, ensuring that Easter is always celebrated around the Northern Hemisphere's spring equinox. Because of the new calendar, several days were skipped, and October 4 was followed by October 15. Today, the Gregorian calendar is the most used calendar around the world.
Gregorian calendar