Solstice

Moments of the year when the sun reaches it's southernmost or northernmost position. There are two Solstices a year: The Summer Solstice is the longest day of the year and marks the beginning of the summer. Similarly, the Winter Solstice is the shortest day of the year, marking the beginning of winter.

Summer Solstices occur in the northern hemisphere on June 20,21 or 22 whereas the Winter Solstice occurs on December 21 or 22. The same days apply for the southern hemisphere, only reversed (i.e., the Summer Solstice occurs in December and the Winter Solstice in June).