Why is 21 of June the shortest day of the year?

Actually, June 21 is typically the longest day of the year in the Northern Hemisphere, not the shortest. This day marks the summer solstice, when the North Pole is tilted closest to the sun, resulting in the longest period of daylight. Conversely, the shortest day of the year occurs around December 21, during the winter solstice, when the North Pole is tilted farthest from the sun.