it is called the summer solstice there are 2 solstices tho 1 is the summer and 1 is winter the summer solstice is the day with longest daylight so it is considered the longest day of the year. take a look at this site it will expllain it better then me http://en.wikipedia.org/wiki/Summer_solstice
if you dont want to go to the site all just copy and paste and u read it
The summer solstice occurs exactly when the Earth's axial tilt is most inclined towards the sun at its maximum of 23° 26' (angle). Though the summer solstice is an instant in time, the term is also colloquially used like Midsummer to refer to the day on which it occurs. Except in the polar regions (where daylight is continuous for many months during the spring and summer), the day on which the summer solstice occurs is the day of the year with the longest period of daylight. Thus the seasonal significance of the Summer solstice is in the reversal of the gradual shortening of nights and lengthening of days. The summer solstice occurs in June in the Northern Hemisphere and in December in the Southern Hemisphere.
North of the Tropic of Cancer (23°26'N) and south of the Tropic of Capricorn (23°26'S), the Sun reaches its highest position in the sky on the day of the summer solstice. However, between the Tropic of Cancer and the Tropic of Capricorn, the highest sun position does not occur at the summer solstice, since the sun reaches the zenith here and it does so at different times of the year depending on the latitude of the observer. Depending on the shift of the calendar, the summer solstice occurs some time between December 21 and December 22 each year in the Southern Hemisphere, and between June 20 and June 21 in the Northern Hemisphere.
Worldwide, interpretation of the event has varied from culture to culture, but most cultures have held a recognition of sign of the fertility, involving holidays, festivals, gatherings, rituals or other celebrations around that time.
It has recently become a popular belief in the United States that the meteorological season of summer begins with the astronomical phenomenon of the summer solstice. However, the period of maximum temperatures in the United States runs from June 4 to September 3.[unreliable source?] Other regions reckon the start of summer to the beginning of the month of the solstice, or even the month preceeding it.
The word solstice derives from Latin sol (sun) and sistere (to stand still).
 Dates and times
The following lists the dates and UTC times of the summer solstice for the early portion of the 21st century.
Year Northern Hemisphere Southern Hemisphere
2000 June 21, 01:48 December 21, 13:37
2001 June 21, 07:38 December 21, 19:21
2002 June 21, 13:24 December 21, 01:14
2003 June 21, 19:10 December 22, 07:04
2004 June 21, 00:57 December 21, 12:42
2005 June 21, 06:46 December 21, 18:35
2006 June 21, 12:26 December 22, 00:22
2007 June 21, 18:06 December 22, 06:08
2008 June 20, 23:59 December 21, 12:04
2009 June 21, 05:46 December 21, 17:47
2010 June 21, 11:28 December 21, 23:38
2011 June 21, 17:16 December 22, 05:30
2012 June 20, 23:09 December 21, 11:12
2013 June 21, 05:04 December 21, 17:11
2014 June 21, 10:51 December 21, 23:03
2015 June 21, 16:38 December 22, 04:48
2016 June 20, 22:34 December 21, 10:44
2017 June 21, 04:24 December 21, 16:28
2018 June 21, 10:07 December 21, 22:23
2019 June 21, 15:54 December 22, 04:19
2020 June 20, 21:44 December 21, 10:02
to read about the winter solstice go here http://en.wikipedia.org/wiki/Winter_solstice