The summer solstice is June 21 and will be the longest day of 2019.

Here's what to know about what happens on the solstice and how various communities around the world celebrate. 

1. It's only in the Northern Hemisphere.

Solstices happen on either side of the equator, so the summer solstice in the Northern Hemisphere happens at the same time as the winter solstice in the Southern Hemisphere.

2. The solstice occurs when the sun is directly over the Tropic of Cancer.

The solstice will occur at exactly 11:54 a.m. Eastern time on June 21, when the sun is at 23.5 degrees north latitude over the Tropic of Cancer.

3. The amount of daylight you will experience depends on where you live.

As you may know, different regions of the United States (and the rest of the Northern Hemisphere) experience differing amounts of sunlight daily. Northern regions will experience more daylight than southern regions. According to climatologist Brian Brettschneider, Americans living in the northernmost states could experience up to 17 hours of daylight on the solstice, while Americans in the southernmost states will experience as little as under 14 hours of daylight. See the regional breakdown on Brettschneider's map here.

4. Stonehenge may play a part in the history of the summer solstice. 

Some historians believe Stonehenge may have been an ancient calendar system and that the summer solstice was the day ancient communities planned their yearly calendar around. Built around 2500 BCE, the structure in England offers a particular view of the sunrise on the solstice, leading historians to believe it may have been an ancient calendar system.

5. People have celebrated the summer solstice for generations.

Many ancient communities hosted midsummer feasts, and in North America, various Native American tribes held ritual dances to honor the sun. In modern day, those with pagan beliefs observe the solstice, with some traveling to Stonehenge in honor of the occasion. In the United States, many people celebrate by holding festivals or parties dedicated to art, music or the environment.