Officially the first day of Summer.

Gledilegt sumar!
The First Day of Summer (sumardagurinn fyrsti) is an annual public holiday in Iceland held on the first Thursday after 18 April. In former times, the Icelanders used the Old Norse calendar which divided the year into only two seasons, winter and summer. Although the climate in late April cannot be considered to be summer-like, after the long winter, Icelanders still celebrate this first day of "summer" with parades, sporting events and organized entertainment, held in various places around Iceland.
(Wikipedia)