As it happens, today is my half-birthday. At least according to some calculations. Because, while you wouldn't think it should be all that difficult to figure out when your half-birthday is (take your birthday, add six months and voilà!), counting by calendar months does leave some people, say, those born at the very end of August, without any half- birthday at all.
So, since there's no need to make life any more unfair than it already is, let's agree that those unlucky folks can count half a year from their actual birthday (182.5 or 183 days, depending on whether it's a leap year or not -- use this handy half-birthday calculator), allowing everyone to have the opportunity to bask in the unadulterated joy that is a half-birthday. Which is even better than an actual birthday because you don't have to get all philosophical contemplating the deep, deep meaning of turning a year older.
On your half birthday, be sure to tell your friends to send you one of these half birthday cards. Bake or buy half a cake and decorate it with half the usual number or candles or, if you prefer, candles cut in half. If you blow them out, exactly half your wishes will come true*
*Of course, à la Half Magic, you could try wishing for twice as much or twice as many as you actually want. But that strikes me as, well, cheating.