Solstice is derived from the Latin words sol (sun) and sistere (to stand still). This is because, on the solstices, the Sun appears to stand still in the sky for a few days before changing direction. The summer solstice occurs on June 20 or 21, marking the longest day of the year in the Northern Hemisphere. The winter solstice occurs on December 21 or 22, marking the shortest day of the year in the Northern Hemisphere.