Memorial Day signals the start of summer for most people. But today – June 21 – is actually the first day of astronomical summer in the Northern Hemisphere.

It’s also the longest day of the year.

On this day, the Northern Hemisphere is angled at the point in its orbit closest to the sun, resulting in the longest day and shortest night. For those in the Southern Hemisphere, today is the shortest day of the year and the start of winter. You can go here to see more of the science behind the solstice.

In the Northern Hemisphere, the summer solstice falls between June 20 and 22 each year. This year, the solstice will occur June 21 at 10:58 a.m. ET (9:58 a.m. CT). After today, the amount of daylight will decrease incrementally through the fall into the winter months.

The Winter Solstice will take place Dec. 21 at 11:27 p.m. ET (10:27 p.m. CT).

You can go here to see the sunrise and sunset times in your area.

Solstice – from the Latin words “sol” for sun and “sistere” meaning “to stand still” – is marked with different traditions around the world. The most famous are Midsummer celebrations, including gatherings at Stonehenge and the lighting of bonfires on hilltops.