December 31 is the last day of the year in the Gregorian calendar, which is the calendar system used by most of the world. It marks the end of December and the conclusion of the annual cycle, leading into the new year on January 1. Many people celebrate this day with various traditions and festivities, often referred to as New Year's Eve. On December 31, people often reflect on the past year, setting goals and resolutions for the upcoming year. In many cultures, it is common to gather with family and friends, enjoy special meals, and participate in countdowns to midnight, symbolizing a fresh start.