Gleðilegt sumar! Happy First Day of Summer!
According to Icelandic tradition, the first day of summer is celebrated the third Thursday of April, not the day of the solstice as celebrated in mainland Europe and the Americas.
Iceland really only experiences two seasons (summer and winter). A runic calendar described and illustrated in the 1643 book "Fasti Danici," shows the year divided into two halves -- summer and winter. The Sagas typically describe the ages of people and animals in terms of "winters."
More about the old Iceland calendar can be found here: https://thornews.com/2015/09/27/the-viking-calendar/