Is National Twin Day an official or public holiday?

No. It is an unofficial observance, not a federal or public holiday, so businesses and schools stay open as usual.

Are identical twins genetically identical?

Not exactly. A study of 381 twin pairs found identical twins differ by an average of 5.2 early-development mutations, and they drift further apart over a lifetime as their epigenetics change.

Why does Nigeria have so many twins?

The town of Igbo-Ora has the highest documented dizygotic twinning rate in the world, about 45 to 50 per 1,000 births. Residents often credit local foods, but the scientific cause is not established.

National Twin Day

Next celebratedFriday, December 18, 2026

An observance on December 18 celebrating twins, other multiples, and the bond between them, alongside the science and culture of twinning.

See how to celebrate

Countdown to the day

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Last updated March 18, 2026 · by the Holiday Calendar TeamHave an update or spot an error?

YEARLY DATEDecember 18

OBSERVED INUnited States

CATEGORYRelationships

SUBCATEGORYSiblings

ORIGIN

Community Origin

FOUNDING ENTITY

Not documented

FIRST OBSERVED

Not documented

HOW THE HOLIDAY CAME TO BE

An observance with no documented author.

No primary or independent source documents who created National Twin Day or when. The December 18 date and its real-world observance are confirmed by institutions that mark it, including the hospital system AdventHealth, but the founding record itself is undocumented.

+ Know the story? Submit a founder Help us complete this holiday

INTRO

Why no two "identical" twins are truly identical

Call them identical and you are already slightly wrong. When scientists at deCODE genetics read the DNA of 381 twin pairs, the average so-called identical pair turned out to differ by 5.2 early-development mutations. Thirty-nine pairs differed by more than a hundred.

The cause is timing. A single embryo splits into two, but it keeps copying its own DNA as it does, and tiny errors creep in during those first cell divisions. Two people who began as one cell walk away from the womb already not quite the same.

National Twin Day, observed each December 18 in the United States, is the day that gap gets its due. This is not a "hug your sibling" date so much as a window into one of biology's strangest experiments, two people built from one blueprint, and never the perfect copies the word "identical" promises.

ORIGINS

Twin Day history

INTRODUCTION

For as long as people have noticed twins, they have struggled to explain them. The deeper history here is not the holiday's, whose own founding goes unrecorded, but the topic's: how twins went from omen to scientific instrument.

The biology came into focus slowly. Monozygotic twins form when one egg, fertilized by one sperm, splits into two embryos within roughly two weeks of conception. Dizygotic twins form when two separate eggs are each fertilized, making fraternal twins no more alike than ordinary siblings.

CHAPTER 01

A town that bet on twins

In 1819 a village called Millsville, Ohio, made an unusual deal. Identical-twin brothers Moses and Aaron Wilcox offered land for a town square and money toward a school, on one condition: the town take their name. Millsville became Twinsburg.

More than a century later the town leaned into its name again. In 1976, as part of the US bicentennial, Twinsburg held its first Twins Days Festival, with about three dozen sets of twins. It grew into the largest annual gathering of twins anywhere.

CHAPTER 02

Twins as a laboratory

Scientists, meanwhile, had realized twins could answer a question nothing else could: how much of who we are is inherited. From 1979 to 1999, the Minnesota Study of Twins Reared Apart, led by Thomas J. Bouchard Jr., tracked 137 pairs separated in childhood.

Its finding unsettled people. Identical twins raised in different homes turned out about as alike as those raised together, evidence that genes shape personality and intelligence more than many had assumed.

CHAPTER 03

A date no one signed

The observance itself has no such paper trail. National Twin Day is real and observed: a hospital system, a local news affiliate, a school district, and a museum have all publicly marked December 18.

But no government record, news investigation, or organization documents who created the day or when. A widely repeated 2019 origin claim traces only to holiday-listing aggregators, with no independent source behind it, so it is left unstated here rather than asserted as fact.

WHY THIS DAY MATTERS

Why We Love National Twin Day

SCIENCE

Two people from one blueprint are biology's cleanest natural experiment.

Comparing identical and fraternal twins is still one of the few ways researchers can separate the effects of genes from environment. The Minnesota study used reared-apart twins to estimate the heritability of traits once thought purely shaped by upbringing.

IDENTITY

They break the systems built to tell us apart

Standard forensic DNA profiling cannot reliably distinguish identical twins, and prosecutions have stalled when no one could prove which twin left a sample. The hardest case for facial recognition is two nearly identical faces.

DEMOGRAPHY

How common twins are keeps changing

The US twin birth rate rose for decades, peaked in 2014, then fell through 2024, with the steepest drops among older mothers. A day about twins is also a marker of how families and childbearing are shifting.

BY THE NUMBERS

National Twin Day by the Numbers

30.1: US twin births per 1,000 in 2024
11%: Drop in US twin rate since 2014 peak
109,195: US twin births in 2024
~12: Global twins per 1,000 births
45-50: Twins per 1,000 in Igbo-Ora, Nigeria

TIMELINE

Timeline

Wilcox twins name a town

Identical-twin brothers Moses and Aaron Wilcox offer land and money to Millsville, Ohio, on the condition it rename itself Twinsburg.

First Twins Days Festival

Twinsburg holds its first Twins Days gathering as part of the US bicentennial, drawing about three dozen sets of twins.

Minnesota twin study begins

Thomas J. Bouchard Jr. launches the Minnesota Study of Twins Reared Apart, which runs 20 years and reshapes the nature-versus-nurture debate.

A world record for the festival

Guinness World Records recognizes Twins Days as the largest annual gathering of twins in the world.

US twin rate peaks

The US twin birth rate hits 33.9 per 1,000 births, the high point before a decade-long decline.

"Identical" proven not identical

A deCODE genetics study of 381 twin pairs shows identical twins differ by an average of 5.2 early mutations.

GOOD TO KNOW

National Twin Day Around the World

Igbo-Ora, Nigeria

the self-styled twins capital of the world, with the highest documented dizygotic rate, about 45 to 50 per 1,000, and its own World Twins Festival

Benin

the highest national twinning rate on record at roughly 27 per 1,000 births, part of a wider West African pattern

East Asia

among the lowest twinning rates in the world, around 5 to 6 per 1,000 births

Twinsburg, Ohio

host of the Twins Days Festival, the world's largest annual gathering of twins, held the first full weekend of August

GOOD TO KNOW

Surprising facts about National Twin Day

Identical twins have different fingerprints

Genes set the broad pattern of loops and whorls, but the fine minutiae form randomly in the womb, so even monozygotic twins can be told apart by print.

The FBI has collected twin biometrics at a festival

Since 2010 an FBI-sponsored West Virginia University team has gathered face images and hand-geometry data at the Twins Days Festival to stress-test facial-recognition systems against near-identical faces.

A few identical pairs really are identical

Even after sequencing 381 twin pairs, the deCODE team found 38 of them with no detectable DNA-level differences at all, the rare cases where the label truly holds.

The US twin drop is steepest among older mothers

From 2014 to 2024 the US twin rate fell 46 percent for mothers aged 40 and older, far more than for younger mothers, per the National Center for Health Statistics.

Identical twins drift further apart with age

As epigenetic markers and DNA methylation shift over a lifetime, monozygotic twins who started nearly identical become least alike when they are old.

GOOD TO KNOW

The main types of twins

Monozygotic (identical)

One fertilized egg splits into two embryos that share nearly the same genome and almost always the same sex.

Dizygotic (fraternal)

Two separate eggs are each fertilized by a different sperm, so the twins are no more alike than ordinary siblings.

Conjoined

A single embryo fails to separate completely, an outcome estimated at roughly 1 in 50,000 to 100,000 births.

Sesquizygotic (semi-identical)

Twins who share all of their mother's genes but only part of their father's, with only a handful of documented cases.

GET INVOLVED

How to Celebrate National Twin Day

Learn what "identical" really means

Read up on why monozygotic twins are not perfect genetic copies of each other. It is the fact most people get wrong about the day's whole subject.

TELL

Tell a twin family's story

Ask twins you know how alike or different they actually are, in habits, health, and personality. Their answers are the nature-versus-nurture question in miniature.

VISIT

Visit or follow a twins festival

Look into the Twins Days Festival in Twinsburg, Ohio, the world's largest gathering of twins. Its history and photos are worth a scroll even if you cannot attend.

READ

Read a real twin study

Spend a few minutes with the Minnesota Study of Twins Reared Apart or the deCODE mutation findings. Each shows how twins reshaped what we know about genes.

Share one fact that corrects a myth

Tell someone that identical twins have different fingerprints and slightly different DNA. A single accurate fact does more for the day than a generic post.

Test your knowledge

How well do you know National Twin Day?

1 / 8

How alike is the DNA of so-called identical twins?

Answer

No. National Twin Day is the December 18 observance, while the Twins Days Festival is a separate event in Twinsburg, Ohio held the first full weekend of August. People often confuse the two, but they fall on different dates and are not connected.