The First Day of Summer, known as Sumardagurinn fyrsti, is a public holiday observed in Iceland every year on the first Thursday after April 18. This celebration, which dates back to the 10th century, marks the beginning of summer according to the traditional Icelandic calendar that divided the year into two seasons: summer and winter. Despite the typically cold weather in late April, the holiday is significant as it was officially declared a public holiday and non-working day in 1921. Celebrations include parades, sporting events, and various entertaining activities throughout the country. It is also one of the official flag days in Iceland, with some people exchanging gifts, although modern customs have shifted to smaller gifts primarily given to children.