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National Twin Day

An observance on December 18 honoring the biological, emotional, and cultural significance of twins and multiples.

Friday

December 2026

Last updated March 18, 2026 · by the Holiday Calendar Team

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YEARLY DATEDecember 18

OBSERVED INUnited States

CATEGORYRelationships

SUBCATEGORYSiblings

ORIGIN

Institutional Initiative

FOUNDING ENTITY

National Day Calendar

FIRST OBSERVED

2019

HOW THE HOLIDAY CAME TO BE

The National Day Calendar proclaimed National Twin Day on December 18 in 2019, establishing a dedicated date for recognizing the unique bond between twins and multiples. The concept of celebrating twins has older roots, including the Twins Days Festival in Twinsburg, Ohio, which has run since 1976.

INTRO

Introduction

PHOTO

More than 110,000 twins are born in the United States each year, a rate of 30.7 per 1,000 live births as of 2023. National Twin Day on December 18 recognizes a group that accounts for about 3% of all births but has outsized significance in science, culture, and family life.

Twins have served as natural experiments in genetics, shaped medical understanding of human development, and become cultural fixtures from mythology to modern media. The observance exists because the twin experience is genuinely different from the singleton one: shared birthdays, constant comparison, and a relationship that starts before either person takes a first breath.

ORIGINS

Twin Day history

INTRODUCTION

Twins have fascinated humans for as long as humans have been recording their observations. In Greek mythology, Castor and Pollux represented the duality of mortality and immortality. In Roman legend, Romulus and Remus founded a city. Across cultures, twins have been treated as omens, blessings, and puzzles. National Twin Day reflects a modern version of that fascination, grounded less in mythology and more in the science and statistics that make twins genuinely remarkable.

The biological mechanics of twinning split into two distinct categories. Identical twins occur when a single fertilized egg divides into two embryos, sharing nearly all their DNA. This happens in roughly 3 to 4 out of every 1,000 births worldwide, and scientists still do not fully understand what triggers the split. Fraternal twins result from two separate eggs fertilized by two different sperm, making them genetically similar to any siblings. Fraternal twins are about twice as common as identical twins and are influenced by maternal age, genetics, and fertility treatments.

CHAPTER 01

The town that took twins seriously

In 1819, identical twins Moses and Aaron Wilcox donated land for a township in Ohio on the condition that it be named Twinsburg. More than 150 years later, in 1976, Twinsburg hosted its first Twins Days Festival as part of its bicentennial celebration. The inaugural event drew just 37 pairs. By 1995, attendance peaked at 2,798 sets of twins, earning the festival a Guinness World Record for the largest annual gathering of twins in the world.

CHAPTER 02

Twins as scientific instruments

The most significant contribution twins have made to human knowledge is in the nature-versus-nurture debate. In 1979, psychologist Thomas Bouchard at the University of Minnesota launched the Minnesota Study of Twins Reared Apart, studying 137 pairs of twins over 20 years. The study's most famous subjects were the Jim twins, James Lewis and James Springer, who were separated at four weeks old and reunited at 39. Despite growing up in different homes, both had married women named Linda, divorced, then married women named Betty. Both named a son James Alan.

In 2015, NASA took twin research to its most extreme setting. Astronaut Scott Kelly spent 340 days aboard the International Space Station while his identical twin Mark Kelly remained on Earth. The NASA Twins Study found that about 7% of Scott's gene expression changed during spaceflight, with most changes reverting within six months of his return.

CHAPTER 03

From science to celebration

The National Day Calendar proclaimed National Twin Day on December 18 in 2019, giving twins a dedicated date distinct from the Twinsburg festival held each August. The observance recognizes not just the biological novelty of twinning but the lived experience: the shared childhoods, the identity questions, and the bond that twin research consistently finds is among the closest human relationships.

TIMELINE

National Twin Day Timeline

Twinsburg, Ohio gets its name

Identical twins Moses and Aaron Wilcox donated land for a new Ohio township on the condition it be named Twinsburg, creating the town that would later host the world's largest twin festival.

Twins Days Festival launches with 37 pairs

Twinsburg, Ohio hosted its first Twins Days Festival as part of its bicentennial celebration. The event would grow into the Guinness World Record holder for the largest annual gathering of twins.

Minnesota twin study begins

Psychologist Thomas Bouchard launched the Minnesota Study of Twins Reared Apart, a 20-year investigation that would study 137 twin pairs to determine the relative influence of genetics versus environment.

Twins Days draws record attendance

The Twinsburg festival peaked with 2,798 sets of twins attending, cementing its Guinness World Record and drawing international media coverage.

NASA sends one twin to space

Astronaut Scott Kelly began a 340-day mission aboard the International Space Station while his identical twin Mark Kelly stayed on Earth, enabling the landmark NASA Twins Study.

National Twin Day proclaimed on December 18

The National Day Calendar officially established December 18 as National Twin Day, creating a dedicated annual observance for twins and multiples.

GET INVOLVED

How to Celebrate National Twin Day

Learn about the science of twinning

The National Human Genome Research Institute explains the genetic mechanisms behind identical and fraternal twins, including why identical twinning remains scientifically unpredictable.

EXPLORE

Explore the NASA Twins Study results

NASA published the full findings of its Twins Study comparing astronaut Scott Kelly's year in space to his identical twin Mark Kelly on Earth. The results reveal how spaceflight affects gene expression, telomeres, and cognition.

Share your twin story on social media

If you are a twin, post a side-by-side photo or a story about what makes your twin relationship unique. The day's hashtag, #NationalTwinDay, connects twin communities across platforms.

READ

Read about the Minnesota twin study

The National Institutes of Health archives research on twin studies that have shaped our understanding of how much genetics influences personality, health, and behavior.

CELEBRATE

Celebrate a twin you know

Send a message to twins in your life acknowledging what makes their bond special. Whether they are young siblings or lifelong partners in navigating the world, recognition from others reinforces a connection that research shows is one of the strongest in human relationships.

WHY THIS DAY MATTERS

Why We Love National Twin Day

The twin experience is genuinely distinct

Twins navigate identity formation differently from singletons, facing constant comparison from peers, teachers, and family. Research shows that the twin bond is among the closest human relationships, but it also comes with unique challenges around individuation, competition, and the experience of losing a twin.

Twin births are common and rising in complexity

The U.S. twin birth rate rose 76% from 1980 to 2009, driven largely by fertility treatments and rising maternal age. Although the rate declined 4% between 2014 and 2018 as single embryo transfers became more common, twins still represent over 110,000 births annually and carry higher risks for preterm delivery and neonatal complications.

Twin studies have reshaped our understanding of genetics

Research using twins, from the Minnesota Study to the NASA Twins Study, has fundamentally changed how scientists understand the interplay between genetics and environment. The finding that identical twins raised apart share striking behavioral similarities has influenced fields from psychology to medicine.

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