I hear idiot television newspeople (but I repeat myself) saying that all the time at this point in the year.
There is a widespread misconception in this country–which extends, I might note, to the makers of most calendars, dictionaries, and encyclopedias–that summer “officially” starts on the day of the summer solstice, June 21 or 22, which is the longest day of the year. Americans also believe (1) that there is some valid scientific reason for doing it that way, and (2) that everybody in the Northern Hemisphere does it that way, and always has.
None of these things is true. So far as I have been able to discover, no scientific or governmental body has ever formally declared that summer starts on the solstice. . . . .
“It isn’t really clear how the astronomical definition [i.e., summer starts on the solstice] got started,” says Kevin Trenberth, a climate researcher at the University of Illinois in Urbana. “Although the sun-earth geometry is clearly the origin of the seasons on earth, it has nothing directly to do with temperature or weather.”
He notes that meteorologists define summer simply as June, July, and August. “For practical purposes, the meteorological definition is the best one, being very closely to the [weather] statistics,” he says.
In fact, it appears that June 1 was accepted as the beginning of summer in the United States until relatively recently.
Go the link to read the part about when summer used to be calculated in Ireland—not May 1, as some might think.
The moment of the summer solstice is 1716 hours UTC, June 21st. Track your solar festivals here.