Some parts of the U.S. are already sweltering, but summer doesn't officially start until this weekend. Sunday marks the summer solstice, or first day of the season, in the Northern Hemisphere. It's also the longest day of the year.

Before you slather on sunscreen and jump into the pool, read these facts about the summer solstice:

1. This year, it falls on June 21. But that's not always true.

The summer solstice happens between June 20 and June 21 every year and formally occurs when the sun reaches its highest point in the sky. This year, the actual time the summer solstice begins is 12:38 p.m., Mental Floss reported. The sun will be over the Tropic of Cancer -- 23 degrees, 27 minutes north latitude. It will be in the sky from 5:25 a.m. to 8:31 p.m. EDT.

2. Where does the word come from?

"Solstice" is a word that means "sun" and "staying," or the sun standing still. While on other days the sun seems to move in the sky, on the solstice it doesn't. In terms of the planet's revolution, the sun is farthest away from the Earth during the North Hemisphere's summer and closest during its winter. Check out your shadow -- around noon on the solstice, it will be the smallest it'll get all year.

3. It's also the Winter Solstice.

At the exact same time summer starts in the Northern Hemisphere, winter starts in the Southern Hemisphere. 

4. In some places, there's constant sunlight.

The so-called "midnight sun" shines all day during the solstice in some parts of Alaska, Canada, Sweden and Finland, according to

5. People have been celebrating it for centuries.

The world has partied for the solstice since ancient times. Stonehenge is home to one of the biggest observations of the summer solstice because legend says the stones were set up to align with the sun on the solstice. As such, thousands of people now gather there to do yoga and worship the sun every year. Yoga is also a solstice tradition in New York City's Time Square. If you can't get there, tune in to the live stream here.