If you happen to live in the Northern Hemisphere, you may want to plan extensive activities to keep dad entertained this Father's Day. That's because June 21st happens to be the start of summer and the longest day of the year. Also known as summer 'solstice' - a Latin word for 'sun stands still', it is the day when the tilt of the Earth's semi axis, in the northern hemisphere, is most inclined toward the sun.