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The Northern Solstice, also called the Summer Solstice or the June Solstice for those in the Northern Hemisphere, is the time and date that each year when the sun is at it’s furthest point north from the equator. In the Northern Hemisphere, it marks the first day of Summer, while in the Southern Hemisphere, it marks the first day of Winter.
The word solstice is derived from the Latin sol and sistere, or sun stand still, because at this time the seasonal movement of the sun stops and reverses direction.
The date of the solstice’s changes from year to year, due to the imprecise alignment of the gregorian calendar to the actual motion of the earth in it’s orbit around the sun. This year it occurs on June 20, at 23:09 UTC.
For more information, the Wikipedia entry on the topic has some great information and diagrams.