31 December in the context of "Saint Sylvester's Day"

In the Gregorian calendar, New Year's Eve refers to the evening, or commonly the entire day, of the last day of the year, 31 December, also known as Old Year's Day. In many countries, New Year's Eve is celebrated with dancing, eating, drinking, and watching or lighting fireworks. Many Christians attend a watchnight service to mark the occasion. New Year's Eve celebrations generally continue into New Year's Day, 1 January, past midnight.

The local time zone determines the advent of the New Year; the first places to welcome the New Year are west of the International Date Line: the Line Islands (part of Kiribati), Samoa and Tonga, in the Pacific Ocean. In contrast, American Samoa, Baker Island and Howland Island (part of the United States Minor Outlying Islands) are among the last.

A common year starting on Monday is any non-leap year (i.e., a year with 365 days) that begins on Monday, 1 January, and ends on Monday, 31 December. Its dominical letter hence is G. The most recent year of such kind was 2018, and the next one will be 2029 in the Gregorian calendar, or likewise, 2019 and 2030 in the Julian calendar, see below for more. This common year is one of the three possible common years in which a century year can begin on and occurs in century years that yield a remainder of 300 when divided by 400. The most recent such year was 1900, and the next one will be 2300.

Any common year that starts on Monday has two Friday the 13ths: those two in this common year occur in April and July.From July of the year in this type of year to September in the year that follows this type of year is the longest period that occurs without a Friday the 13th, unless the following year is a leap year starting on Tuesday, in which case the gap only 11 months, as the next Friday the 13th is already in June.

The 21st century is the current century in the Anno Domini or Common Era, in accordance with the Gregorian calendar. It began on 1 January 2001 (MMI), and will end on 31 December 2100 (MMC). It is the first century of the 3rd millennium.

The rise of a global economy and Third World consumerism marked the beginning of the century, along with increased private enterprise and deepening concern over terrorism after the September 11 attacks in 2001. The NATO intervention in Afghanistan and the United States-led coalition intervention in Iraq in the early 2000s, as well as the overthrow of several regimes during the Arab Spring in the early 2010s, led to mixed outcomes in the Arab world, resulting in several civil wars and political instability. The early 2020s saw an increase in wars across the world, including a major escalation of the Russo-Ukrainian war and the Gaza war. Meanwhile, the war on drugs continues, with the focus primarily on Mexico and the rest of Latin America. The United States has remained the sole global superpower, while China is now considered to be an emerging superpower.

A common year starting on Thursday is any non-leap year (i.e. a year with 365 days) that begins on Thursday, 1 January, and ends on Thursday, 31 December. Its dominical letter hence is D. The most recent year of such kind was 2015, and the next one will be 2026 in the Gregorian calendar or, likewise, 2021 and 2027 in the obsolete Julian calendar, see below for more.

This is the only common year with three occurrences of Friday the 13th: those three in this common year occur in February, March, and November. Leap years starting on Sunday share this characteristic, for the months January, April and July. From February until March in this type of year is also the shortest period (one month) that runs between two instances of Friday the 13th. Additionally, this is the one of only two types of years overall where a rectangular February is possible, in places where Sunday is considered to be the first day of the week. Common years starting on Friday share this characteristic, when Monday is considered to be the first day of the week.