21st June is the longest day in the northern hemisphere, surprisingly its also celebrated as the international yoga day. This day is called Summer Solstice and The sun reaches its northernmost point in the sky. This is called the summer solstice in the Northern Hemisphere and in the Southern Hemisphere, it will be the shortest day of the year, called the winter solstice. This situation is reversed on Dec. 21, when the sun is at its southernmost declination. People in the Northern Hemisphere call this the winter solstice, and those in the Southern Hemisphere call it the summer solstice.