The start of summer astronomically falls on the summer solstice, and in Finland it’s today; it marks the moment when the Earth’s poles are at their maximum tilt toward the sun.
It commemorates the longest day of the year! (It’s the opposite of the December solstice, which marks the longest night and shortest day.)
Geographically this means that the Sun is directly above the Tropic of Cancer in the Northern Hemisphere and that the path of the Sun in the sky is farthest north. (The Tropic of Cancer passes through Mexico, northern Africa, the Middle East, India and China.)
In southern Finland, the day is about 19 hours long.