A solstice is a solar event marking one of the two annual extremes of the sun’s appearance of height in the sky/distance from the horizon. During the Summer Solstice, the sun appears at its highest point for the year and the day of the Summer Solstice receives the longest period of sunlight for the year. On the day of the Winter Solstice, the sun appears lowest in the sky and the day receives the least amount of sunlight.
In the Northern Hemisphere, the Winter Solstice takes place in December, on the 20th, 21st or 22nd, rarely the 23rd. In the Southern Hemisphere, the Winter Solstice takes place in June, on the 20th-23rd.
In the Northern Hemisphere, the Summer Solstice takes place in June, sometime from the 20th-23rd of the month. In the Southern Hemisphere, the Summer Solstice takes place in December.
Both of these occasions see widespread observance throughout the world, though the Winter Solstice tends to get quite a bit more attention than the Summer Solstice.