National Twin Day is observed worldwide every December 18 to celebrate the unique bond shared by twins, who are siblings produced from the same pregnancy. The day was proclaimed by the National Day Calendar in 2019. It honors both identical twins, who develop from a single fertilized egg and share nearly identical genetic makeup, and fraternal twins, who develop from separate eggs fertilized by different sperm. People celebrate by spending time with their twin siblings, sharing memories and photos on social media using the hashtag #NationalTwinDay. The celebration is open to everyone interested in twins, allowing them to acknowledge their favorite twins, learn facts about famous twins, or enjoy media featuring twins. It is important to note that National Twin Day is distinct from National Twins Day, which is celebrated on the first full weekend of August during the Twins Days festival in Twinsburg, Ohio.