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The Longest Day on the Earth
--On or around June 21 is the longest day on the earth. 
--The Equator receives 12 hours of daylight, 
--When the North Pole and areas North of 66 degree and 30 degrees (Arctic Line) receive 24 hour daylight; there is 24 hour darkness at the South Pole and areas south of 66 degree and 30 degrees (Antarctic Line).

--On this day (the longest day) the Summer starts in Northern hemisphere, while Winter starts in Southern hemisphere.
While it is the longest day in Northern hemisphere, that is the shortest day in the Southern hemisphere.

It is worth noted here that the Sun does not rise on this day at the earliest in the morning or sets at the latest in the night. In fact the earliest time of sunrise or sunset differs from place to place. 

Anchorage (Alaska, USA) has the longest day on June 21. The Sun rises on this day at 4.20 am and sets at 11.42 pm. The earliest sunrise lasts only for 7 days - from June 15 through June 23, and the latest sunset lasts from June 21 to June 26. Nairobi (Kenya, Africa), which is exactly 1 degree 47' South of the Equator, experiences exactly 12 hours of daylight and 12 hours of night on June 21 - sunrise at 6.33 am and sunset at 6.33 pm; while the longest day on December 21 - sunrise at 6.22 am and sunset at 6.37 pm.

Why Seasons Happen on the Earth?
People say that seasons are caused by the Earth being closer to the Sun in the summer and far in the winter. This is not so. The Earth is actually farther from the Sun when it is summer in the Northern Hemisphere. In fact seasons are the result of the Earth being tilted on its axis by 23.5 degrees. As the Earth orbits the Sun, different parts of the world receive different amounts of direct sunrays and different amount of tilted sunlight. In July, the Northern Hemisphere is tilted towards the Sun giving longer days and more direct sunlight; while in January, it is tilted the other way that is why giving the shorter days and not much direct sunlight to the Northern Hemisphere. The seasons are reversed in the Southern Hemisphere, which is tilted towards the Sun in January and away from the Sun in July. In tropical areas of the world, there is no noticeable change in the amount of sunlight, that is why there the day and night period remains the same.

Direct sunlight gives more heat (while the Sun is normally much far from the Earth),
while tilted sunlight gives less heat (while the Sun is normally nearer to the Earth),
That is why summer and winter are not the outcome of the Sun being near or far but because of the direct or tilted sunrays.