June 21 is always the 172nd day of the year, unless it's a leap year in which it is the 173rd day of the year. This is on a Gregorian calendar, which leaves 193 days remaining until January 1st, where a new year beings. June 21 marks the summer solstice in the northern hemisphere and the winter solstice in the southern hemisphere. That means that June 21 is typically the day of the year year with the longest hours of daylight in the northern hemisphere and the shortest in the southern hemisphere. You can tell already that June 21 is an important date to mark for several reasons. Here are several events that have happened on June 21 in the past: