The first day of summer is determined by the tilt of the Earth’s axis as it rotates around the sun. Photo courtesy space.com

June 20. Tuesday marks the first day of summer, and the longest day of the year. The summer solstice — is observed on either June 20, 21 or 22, but this year, it falls on June 21.

The timing shifts because it depends on when the sun reaches its northernmost point from the celestial equator.

In the Southern Hemisphere, it’s the opposite: the June solstice marks the astronomical start of winter, when the sun is at its lowest point in the sky.

Online fun for kids

Find summer activities for the kids here.  You can save it for a rainy summer day.

Another reason June 21 is special

International Yoga Day is celebrated across the globe on June 21 making today International Yoga Day Eve.