Yes, December does have a 31st day in the standard Gregorian calendar used worldwide, and that date is December 31, commonly known as New Year’s Eve in many countries.

Basic calendar fact

  • December is the 12th month of the year and always has 31 days.
  • December 31 is the last day of the year (365th day, or 366th in a leap year).

What happens on December 31?

  • December 31 is widely celebrated as New Year’s Eve, with parties, fireworks, and countdowns to midnight.
  • Many cities, such as New York with the Times Square ball drop, have famous annual celebrations on this date.

Fun context

  • Because it’s the year’s final day, lots of people use December 31 to reflect on the past year and make resolutions for the next one.
  • Different cultures have unique traditions for the night of December 31, but they all center on welcoming the coming year.

So if you are planning something “on the 31st of December,” that date definitely exists and is a major calendar and cultural milestone each year.

TL;DR: Yes, there is a 31 in December; it’s December 31, the final day of the year and widely celebrated as New Year’s Eve.

Information gathered from public forums or data available on the internet and portrayed here.