In the Northern Hemisphere, the longest day of the year occurs on the summer solstice (when the northern pole of earth's axis is tilted toward the sun). The sun is in the highest position in the sky as seen from the North Pole. This occurs between June 20 and 22 each year. The hours of sunlight each day gets progressively shorter each day until the winter solstice (the shortest day of sunlight). On this day the sun is in the lowest position in the sky (closest to the horizon). It occurs between December 20 and 22 each year. A solstice is not actually a day, but a moment of a day, the exact moment the sun reaches its highest or lowest point in the sky. From the winter solstice, the hours of sunlight gets progressively longer each day until the summer solstice.
The opposite occurs in the Southern Hemisphere.
This change in position of the sun relative to the horizon changes the location of the place in the continental US that is first to see the sunrise. You can check out my Hub on this.