The birthday of Princess Isabella of Denmark is observed in Denmark every April 21. This occasion is marked by the official flag-flying day, where the national flag is displayed prominently on public buildings and ships throughout the country. Princess Isabella, born on April 21, 2007, to Crown Prince Frederik and Crown Princess Mary, is celebrated for being the first Danish princess to have her birthday honored with a 21-gun salute, a tradition that previously varied based on the sex of the royal child. Her birthday celebrations include various official events, such as a birthday performance attended by the Royal Family and young people from across Denmark. In 2025, when she turned 18, her birthday was officially recognized as a flag-flying day. This event is part of the broader customs associated with royal birthdays in Denmark, which often include public festivities and acknowledgments of the royal family's lineage and contributions.