The term 'longest or shortest day' refers to the solstice, which is an astronomical event that occurs twice a year when the Sun reaches its highest or lowest point in the sky. The solstice marks the longest day of the year in the Northern Hemisphere and the shortest day of the year in the Southern Hemisphere. During the summer solstice, the Sun is at its highest point in the sky, and during the winter solstice, the Sun is at its lowest point in the sky.