Summer officially begins, astronomically, with the summer solstice, which this year will occur on June 20, at 20:51 UTC (4:51 PM EDT). Yes, we are used to the solstice being on June 21st, but not this year. In fact, the last time the solstice was this early was when George Washington was the US president!
An explanation of this phenomena starts out pretty dense and confusing, but it's actually pretty simple. First, we get an explanation of the solstice as it pertains to earth's orbit. Then there is an explanation of how calendars work, with a history that we've covered here at Neatorama a few times. It all boils down to leap years, when we add an extra day to the year to make our calendars fall more in line with the earth's orbit around the sun. An actual year is 365.242189 days, so we add an extra day every four years to straighten it up. Every leap year makes the solstice earlier, but then non-leap years make it later again. But you can see by that fraction of a day that more adjustments were needed after a few hundred years.
In the late 16th century, more rules about leap years were added, so that century years (1800, 1900, etc) are not leap years, unless that century year is divisible by 400. The year 2000 was a century year divisible by 400, so it was a leap year, the first century leap year since the Gregorian calendar was finalized. Therefore, we have now had a long string of leap years every four years, making the solstice earlier than ever. This is all explained mathematically at Big Think.
(Image credit: Jfishburn)