Well, only once unless one happened to be up at 12:35 AM this morning, in which case it will come twice.
Other sites are pointing out that, at 4:56 this afternoon, the time will be 4:56 07/08/09. This, of course, will be another momentous, once-in-a-lifetime event that will only come once in human history and, much like December 31, 1999 or the famed "eleventh hour of the eleventh day of the eleventh month," it is something that humanity must celebrate together.
Of course, these "once in a lifetime moments" aren't really all that uncommon. For example, at 1:23 AM on April 5, 2006, the sequence was 1:23 04/05/06. That is, of course, unless one lives in a country in which the year comes first, in which case it was 1:23 06/04/05. Then again, in Britain, one puts the day before the month, thus: 1:23 05/04/06.
Then again, as the more philosophical among us might be inclined to point out, every moment occurs only once in a lifetime. In that context, anything that can inspire a few people to recognize the significance of one of the many seconds in their lives can't be all bad.
I can't claim to be immune to these attractions. For example, I remember being incredibly excited about August 8, 1988: when transcribed in Roman numerals, that date had more digits than any other day in the twentieth century. On that date -- VIII VIII MCMLXXXVIII -- I was doing some work in my high school, and decided to write the digits on a chalk board. A few weeks later, when the school year began, they were still there.
In retrospect, this seems almost insanely antiquated. I shared the moment with a couple of friends who happened to be in the school building and experienced a slight thrill when I later saw the numbers on the chalkboard. However, the system that gave August 8 such significance had been largely out of use for centuries, and our method of conveying the information -- a chalkboard -- was painfully old fashioned.
By comparison, today's event is based in digital clocks, a solidly 20th century technology, and is being promulgated through social networking media, arguably the premier communication tool of the 21st century. The internet makes it possible for people who are interested in such things to share the moment with hundreds of other time trivia fans. One could almost certainly find a live feed of a huge digital clock somewhere; alternately, it's always possible to visit the national clock. For the craftier among us, the Industrious Clock has handwritten days, months, hours, and minutes that give a certain old fashioned thrill to the moment.
As social networking makes it easier to enjoy historical moments with others, it gets harder to differentiate between real events and pseudo-events. In a larger context, however, that may be beside the point: whether 12:34 this afternoon will be a momentous instant in the annals of human history or simply an opportunity for a few time geeks to share a laugh, the fact that a few number fans are able to coalesce and promote this message shows just how far human media has come. In a larger context, today shows us one key thing: social networking is rapidly transforming from a tool for living life into the raw stuff of life itself.