In ancient times, the date of the June solstice was an important source to help people manage their calendars and organize when to plant and harvest crops. This time of year was also a traditional month for weddings. Some societies invested much effort to establish the length of the year. 
June is the month when summer starts and daylight is longest in the Northern Hemisphere. The longest day is on or around the 21st of June, that is known as the Summer solstice.
June is the month when winter starts and daylight is shortest in the Southern Hemisphere. The shortest day is on or around the 21st of June, that is known as the Winter solstice.
The other month when there is a solstice is December.