Today is Sumardagurinn fyrsti, or the first day of summer, in Iceland. This public holiday is observed every year on the first Thursday of April after the 18th.
The holiday is a throwback to the old Icelandic calendar, which was divided into two halves—six months of nattleysi (nightless days, or summer), and six months of skammdegi (short days, or winter). The first day of summer is the first day of the month of Harpa, which is also the first month of the year according to the old Icelandic calendar.
The holiday may seem particularly ironic, given that the weather is often quite cold in Iceland in April, but tradition holds that cold weather is actually lucky on this day. That is, if the temperature drops below freezing on the eve of the holiday, summer and winter are said to “freeze together,” which bodes a good, warm summer. Read what you will, then, into the fact that temperatures around the country last night were close to, but not quite freezing, ranging from 3-7°C [37-44°F]. But at least the day off will be a reasonably pleasant one for much of the country, with sunshine and temperatures of 6-12°C [43-54°F] in the Westfjords, the north, and the east of Iceland. City dwellers, on the other hand, can count on a fairly warm day, 11°C [52°F], but will have to make due with overcast skies.
Be sure to check opening hours before venturing out today—many businesses will have shorter hours or closures in observance of the holiday. Strætó will run according to a Sunday schedule. The Bonús grocery store will have normal opening hours, as will Nettó, Krónan, and Hagkaup. Swimming pools will also be open regular hours.