National Twin Day Date in the current year: December 18, 2024

Twins are two offspring produced by the same pregnancy. They can be either identical, meaning that they develop from one egg that forms two embryos after being fertilized, or non-identical (fraternal), meaning that each of the twins develops from a separate egg fertilized by its own sperm cell.

Identical twins don’t just look alike; they are nearly (but not one hundred per cent) identical genetically and always have the same biological sex unless a mutation has occurred during fetal development. However, they do have different fingerprints. Identical twins are very rare; they account for about 0.3% of all births.

Unlike identical twins that develop from the same egg fertilized by the same sperm cell, fraternal twins are, essentially, a pair of regular siblings who just happen to share the womb. They share about half of each other’s DNA, which is comparable to siblings that are conceived and born at different times. Fraternal twins may be of opposite biological sexes and may look similar like any other siblings, which is not to the point of confusion.

Twins are often closer than other siblings by virtue of being the same age and growing up together. About half of the young twins develop their own private language that is spoken and understood just by the two of them; this phenomenon is called cryptophasia, or twin talk. However, twins may grow apart as they become older, since even identical twins can have very different personalities.

National Twin Day was proclaimed by the National Day Calendar in 2019. If you’re a twin, celebrate this amazing holiday by spending the day with your sibling and sharing your best twin memories and photos on social media with the hashtag #NationalTwinDay.

National Twin Day is not only for twins though. The celebration is open to everyone who is fascinated by twins, as long as their fascination doesn’t come across as creepy. You can give a shout-out to your favorite twins on social media, learn interesting facts about famous twins, and read a book or watch a movie about twins or featuring twins (some of our favorites include It Takes Two, The Parent Trap, The Shining, Twitches, anything with Mary-Kate and Ashley Olsen, and, of course, the Star Wars movies with Luke and Leia, and the Harry Potter movies with their amazing duo of Fred and George Weasley).

National Twin Day should not be confused with National Twins Day, which is celebrated annually on the first full weekend of August, coinciding with the Twins Days festival in Twinsburg, Ohio. Held every summer since 1976 (with the exception of 2020 because of, you know, the pandemic), the festival draws thousands of pairs of twins from all over the United States and other countries, including Australia, Belgium, Brazil, Canada, China, Germany, Ghana, India, Ireland, Italy, Japan, New Zealand, Nigeria, the UK, and more.