The start of summer means longer days, shorter nights, and hotter weather for the Northern Hemisphere, and the season itself begins with the longest day of the year: the summer solstice.

Every year, the day of the solstice changes, usually falling somewhere between June 20 and June 22 for countries in the Northern Hemisphere. The summer solstice is not based on a certain date or time in the year; instead, it’s based on where the Earth is in relation to the sun. The Earth rotates on an axis that is tilted at 23.5 degrees. As the Earth revolves around the sun, it is exposed to more or less light, causing both the changes in daylength and the changes in season. The summer, or June solstice as it’s sometimes called, occurs when the Earth’s north pole is tilted more towards the sun than at any other time in the year, according to National Geographic.

This year’s summer solstice is on June 21st and will begin when the sun rises at 5:37 a.m. and will end at 8:07 p.m., when the sun sets. The solstice will last 15 hours and 30 minutes.

A common misconception about the summer solstice is that is also the hottest day of the year, but that’s not true. While the earth does receive more sunlight during the solstice than any other day, the heat is absorbed by the ocean and Earth, where it remains trapped there for a few weeks. The hottest day of the year, sometime in July or August, is caused by the release of this stored heat. according to The Washington Post.

The longest day in the Northern Hemisphere not only marks the start of summer, but in some countries like Iceland and England, the day also marks a cause for celebration, according to TIME.

For centuries, the summer solstice has been celebrated by cultures worldwide, according to TIME and National Geographic. The Egyptians, for example, built the pyramids so that during the summer solstice, the sun appears to be setting between them from the Sphinx’s point of view.

Likewise, people have been visiting Stonehenge for generations to celebrate the longest day. The site is known for its alignment with the sun’s movements, and is assumed to have been a place of ancient worship. Today, people visit the old site to dance, listen to music, do yoga, and simply celebrate the natural wonder of the solstice.

In America, a solstice celebration can be found in Fairbanks, Alaska during the Midnight Sun baseball game. The tradition has been taking place since the 1960’s, and the game begins at 10:30 p.m. and goes through midnight. Since Alaska gets 22.5 hours of sunlight during this time of year, the game is played out in the open.

Celebrations like this take place all over the world, from Pagan celebrations and bonfire jumping in Russia to a mass yoga session held in India. While the day itself may be less important than it was in the past, the mass celebrations help to keep the day exciting.

Even though the days start getting shorter again after the solstice, the summer is just getting started.