Before explaining which is the year’s longest day, it should first be mentioned that while we are talking about the longest day, we are not talking about the entire day, because that is fixed to contain twenty four years each and every day of the year. By longest day we mean the day on which daylight is visible for the longest duration in the whole year. Also, following the longest day, the shortest night of the year would also come naturally. The day is called the summer solstice and depending on which Hemisphere of the earth you are talking about, the date would vary accordingly. If you want to know on which date the summer solstice or the longest day falls every year in the Northern Hemisphere, the answer would be the 20th or the 21st June. If you are asking the same question about the Southern Hemisphere, then the answer to that question would be 21st or 22nd December. In the Northern Hemisphere, the sun would be perpendicular to the Tropic of Cancer on this day and the axial tilt would be 23°30′. The concept of the solstices and the equinoxes are such that if one Hemisphere experiences the summer solstice, the other experiences the winter solstice at the exact same time. For example, when the Northern Hemisphere will experience the longest day on the 21st of June in 2011, the Southern Hemisphere will experience the winter solstice at the same time and vice versa.
Apart from marking the longest day of the year, the summer solstice also marks the onset of the summer months, although, this is only an astronomical point of view and summer may arrive earlier or later than the date, depending on a number of other factors. Meteorologists consider 1st June as the first day of summer as it makes data recording and interpreting easier. Culturally, this day has been marked from the ancient of times (approximately, as earlier, they did not have the technology to calculate the exact date) as a sign of fertility and life, because after the frozen winter, life that blooms in spring, thrives in summer.